• Open Access

    Trained-immunity and cross-reactivity for protection: insights from the coronavirus disease 2019 and monkeypox emergencies for vaccine development

    Amanda Izeli Portilho 1,2
    Elizabeth De Gaspari 1,2*

    Explor Immunol. 2023;3:276–285 DOI: https://doi.org/10.37349/ei.2023.00102

    Received: February 10, 2023 Accepted: April 11, 2023 Published: August 24, 2023

    Academic Editor: Jochen Mattner, FAU Erlangen-Nürnberg and UKER, Germany

    This article belongs to the special issue Mucosal Immune Cells - Border Protection or Entrance Gate?


    The emergence and re-emergence of pathogens is a public-health concern, which has become more evident after the coronavirus disease 2019 (COVID-19) pandemic and the monkeypox outbreaks in early 2022. Given that vaccines are the more effective and affordable tools to control infectious diseases, the authors reviewed two heterologous effects of vaccines: the trained immunity and the cross-reactivity. Trained immunity, provided by attenuated vaccines, was exemplified in this article by the decreased the burden of COVID-19 in populations with high Bacille Calmette-Guerin (BCG) coverage. Cross-reactive responses were exemplified here by the studies which suggested that vaccinia could help controlling the monkeypox outbreak, because of common epitopes shared by orthopoxviruses. Although modern vaccination is likely to use subunit vaccines, the authors discussed how adjuvants might be the key to induce trained immunity and improve cross-reactive responses, ensuring that heterologous effects would improve the vaccine’s response.


    Coronavirus disease 2019, monkeypox, trained-immunity, cross-immunity, adjuvants


    The emergence and re-emergence of pathogens is an important problem for public health. The current list of the World Health Organization points the recent coronavirus disease (COVID) pandemic and the outbreaks of Ebola and monkeypox as public health emergencies [1]. Several factors explain the emergence and re-emergence of pathogens; however, some of them are unlikely to change—as the globalization and the environmental changes already promoted by men, suggesting that the health systems should adapt and respond to such problems [24].

    Another issue that plays an important role in the re-emergence of pathogens is the low vaccine coverage. Recently, polio cases have emerged after years of elimination of the disease in the United States [5], Israel, and Ukraine [6]; thus, there is a serious risk of re-emergence of poliomyelitis in other countries, such as Brazil and Venezuela [7, 8]. Other examples of immune-preventable diseases which reemerged after the decline of vaccinations are measles, mumps [9], and the yellow fever [10].

    During the COVID pandemic, different authors described how the prevalence of the disease was reduced in populations with high coverage rates of the Bacille Calmette-Guerin (BCG) vaccine [11]; and the same was observed for influenza, lower respiratory infections, and overall children mortality [12]. The most accepted explanation is the trained immunity, which would improve the immune response to different pathogens [13]. Meanwhile, after monkeypox emergency, some studies reviewed the potential efficacy of vaccinia to protect against the pathogen, based on cross-reaction between orthopoxviruses [14, 15]. Trained immunity and cross-reactivity are heterologous benefits of vaccines—that means, they confer some level of protection apart from the pathogen-specific protection [16].

    Here, we summarize how these two different heterologous effects of vaccines may contribute to fight infectious diseases, using the recent examples of COVID-19 for trained-immunity and monkeypox for cross-reactivity. In addition, we discuss how new, modern vaccines could achieve similar effects: even though subunit differ from the attenuated vaccines according to immunogenicity, trained-immunity induction and cross-reactivity potential, adjuvants based on pathogen-associated molecular patterns (PAMPs), recognized by pattern-recognition receptors (PRRs), could be explored for improving the heterologous benefits. The chart below illustrates these points, which will be reviewed in this article (Figure 1).

    Contributions of PAMPs adjuvants to subunit vaccines. While subunit vaccines are safer, presenting less reactogenicity, attenuated vaccines are more immunogenic and likely to enhance cross-reactivity (because there are more antigens available) and trained-immunity (because of PRRs recognition). PAMPs, such as outer membrane vesicles (OMVs) from Gram negative bacteria, lipopolysaccharide (LPS), and other PRR agonists activate the innate response, similarly to attenuated vaccines, potentially working on trained immunity; thus, they might confer cross-reaction because they are expressed by different pathogens. Finally, they enhance the immunogenicity of subunit antigens, which are safer and preferable for special populations, such as pregnant women, immunosuppressed patients, children, and the elderly. CpG: cytosine-phosphate-guanine

    Trained immunity benefits

    Trained immunity is defined as the functional reprogramming of innate cells in response to PRRs unspecific stimuli. Trained immunity happens on a long-term basis and allows the immune system to respond with more or less intensity, adapting itself to the context and reacting adequately upon a second stimuli [17]. As it is known, the innate activates the adaptative immunity, secreting cytokines and chemokines to recruit effector cells and modulate the overall response [18]. This type of “memory” of innate cells is crucial for the individual homeostasis, establishing patterns for activation or tolerance [19]. Considering anti-viral response, trained immunity results in increased receptors expression, enhanced phagocytic and killing capacity and production and secretion of adequate cytokines for anti-viral response [20].

    Certain live-attenuated vaccines were shown to induce heterologous effect, which has been attributed to trained immunity: smallpox-vaccinees were less likely to be hospitalized by different infectious diseases [21]; measles and yellow fever immunization were associated with reduced carriage of Haemophilus influenzae and pneumococci [22]; administration of measles-mumps-rubella (MMR) vaccine was associated with reduced hospitalization [23] and combined vaccination with oral polio and BCG resulted in decreased children mortality [24]. BCG has been extensively studied for its heterologous effects, which are the reduced prevalence of other mycobacterium infections, as leprosy [25], prevention of hospitalization by respiratory infections and sepsis [26], infant survival [27], protection against yellow fever attenuated virus [28] and, more recently, reduced COVID-19 fatal cases [29].

    The trained immunity phenomena highlight how vaccines present several benefits, contributing to non-specific protection from different pathogens and regulation of tolerance mechanisms [16]. Unfortunately, the increasing anti-vaccine movements and the decay in vaccine coverage in various countries are preventing people from obtaining these benefits and, during the pandemic, the problem worsened [3032].

    Cross-reactive responses

    Vaccinia is an orthopoxvirus, which had been used for attenuated vaccines that allowed the eradication of smallpox. As soon as the monkeypox outbreak started, there were questioning about the efficacy of vaccinia to prevent the infection [33]. However, it should be noted that, after smallpox eradication, the production of such vaccines became limited: now, there are modified vaccinia Ankara (MVA), nonreplicating; ACAM, replicant competent and LC16, minimally replicating [34]. Bioinformatics analysis predicted that cross-reactivity was likely to happen because of similar sequences in vaccinia and monkeypox viruses [35].

    Immunization with vaccinia induces a broad response characterized by neutralizing antibodies with different specificities, which supports the cross-reactive response for different poxviruses [36]. A study found memory B cells in volunteers vaccinated with vaccinia virus over 40 years ago and, even though these cells were rare, the group managed to characterize a monoclonal antibody which was promising as therapeutic and prophylactic treatment against poxviruses [37]. In addition, a serologic investigation verified circulating neutralizing antibodies in 33–53% of people older than 45 years, who were likely to have been vaccinated against smallpox [38].

    It was described that neutralizing antibodies correlate with protection against poxvirus [39, 40]; however, CD4+ cells are just as important, provided that they support antibody secretion, seem to be longer-lived than CD8+ cells and major histocompatibility complex (MHC)-II epitopes are likely to present cross-reaction with different poxviruses [4042]. Moreover, animal studies highlighted previously that cytotoxic response triggered by vaccinia seems to be related to the administration site: scarification would confer a better CD8+ response, because of skin-resident T cells, which would be better than injection for protection upon dermal challenge [43, 44]. Furthermore, mice studies found that vaccinia inoculation induced trained natural killer (NK) cells, which protected the animals depleted from T and B lymphocytes from viral challenge, suggesting how vaccinia immunization might result in polarization of different arms of the immune response, which are likely to protect the recipients [45].

    From human, real-world evidence, an observational study estimated the vaccinia efficacy against monkeypox to be 79%, although it has not been peer-reviewed yet [46]. A study conducted 5 years after discontinuation of smallpox vaccination predicted that the population would be 85% protected against monkeypox [47]. However, as Fine and collaborators [47] highlighted in the manuscript, the waning immunity over the years is likely to decrease this level of protection. Indeed, a serological survey which comprised European, South American, Asian, and African samples verified low neutralizing antibodies in people vaccinated decades before, as well as in unvaccinated volunteers [48]. Another study found 400 monkeypox cases in 7,339 vaccinees who received one dose of MVA, highlighting that the two-dose regimen is required for better efficacy [49]. On the other hand, another study found that when unvaccinated people are compared to people vaccinated with one dose of a non-replicant vaccinia virus, they present a 7.4 higher risk of getting infected by monkeypox; when compared to people vaccinated with two doses, the risk increases to 9.6—showing that even one dose can confer some level of protection, what might be important to respond to outbreaks, when there is a limited vaccine supply [50]. Altogether, these studies support the importance of studying immunization strategies to control orthopoxviruses outbreaks, whether using existing smallpox vaccines or searching new ones.

    To note, immunosuppressed patients and pregnant women present poorer outcomes to monkeypox infection [33] and, during the COVID-19 pandemic, it was pointed out how these groups were underrepresented in clinical trials, hampering the vaccination policies for these populations [51]. All that considered, it would be important investigate subunit vaccines, which are safer options for these groups [52]. Cross-reactive epitopes of vaccinia virus were identified previously [42, 53], intranasal-delivery of recombinant vaccinia antigen was promising for mucosal protection [54] and, compared to DNA, recombinant proteins were more immunogenic and protective [55]. Despite the lack of studies comparing protein antigens to the attenuated virus, the search for subunit candidates for poxviruses vaccines should be encouraged.

    Could subunit vaccines mimic trained-immunity and enhance cross-reactivity?

    Attenuated and inactivated vaccines present several PAMPs, which are likely to promote trained immunity, consequently conferring heterologous effects for these vaccines [16, 56]. Even though subunit vaccines are safer, the scientific community can explore the trained immunity benefits of PAMPs investigating them as adjuvants [19, 52]. In addition to enhanced immunogenicity and targeting of trained immunity, PAMPs are affordable options for developing countries, allowing them to establish their own manufacturing, consequentially improving vaccine coverage [57, 58].

    Excellent revisions described the benefits of PAMPs as adjuvants [57, 59], some of them focused on COVID-19 [60, 61]. The combination of PAMPs or chemically synthetized PRRs agonists to other adjuvants and delivery systems is another strategy which has been studied, not only to enhance immunogenicity but also to modulate the immune response [59].

    Scaria et al. [62] observed a T helper 1 (Th1)-biased immune response when conjugating the receptor binding domain (RBD) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to detoxified diphtheria toxin (EcoCRM®) and Adjuvant System (AS) 01 [a liposome-based adjuvant which contains monophosphoryl lipid A (MPL), a toll-like receptor (TLR)-4 agonist, and the saponin QS-21]. The quantity and functionality of antibodies was robust even when a low antigen concentration was used. In Jangra et al. [63], the subunit 1 of spike SARS-CoV-2 protein was complexed with a nanoemulsion containing in vitro transcribed (IVT) messenger RNA (mRNA) adjuvant, a retinoic acid-inducible gene I (RIG-I) agonist, and induced functional antibodies, which could neutralize other SARS-CoV-2 variants and protect naive mice from virus challenge. The L1 envelope protein of vaccinia adsorbed in alum induced the higher neutralization percentage when combined with CpG, a TLR-9 agonist [64]. Although most studies exploring adjuvants combinations use animal models, the COVID-19 vaccine Soberana 01 is on clinical trials. It is composed by recombinant RBD adsorbed in alum and adjuvanted by N. meningitidis OMVs and induced antibodies which neutralized the D614G variant, which presents enhanced transmission. Importantly, most of the adverse effects related were local reactions with mild intensity [65]. Another example from clinical trials is the vaccination with Cervarix® (GlaxoSmithKline), which employs AS04 as adjuvant (composed of MPL adsorbed in alum) and decreased human papillomavirus (HPV) infections apart from types 16 and 18, probably due cross-reactive antibodies [66].

    Reactogenicity concerns

    An important concern about attenuated and inactivated vaccines is the reactogenicity, and the same could happen with PAMP adjuvants [52]; however, several PAMP-containing adjuvants have been tested in clinical trials, presenting acceptable adverse events, as summarized in Table 1.

    Adverse events reported in clinical trials using PAMP-based adjuvants

    AdjuvantCompositionVaccinesAdverse events
    AS01MPL and saponinSUIVs—influenza (NCT03275389) [67]Injection site pain, fatigue, headache, myalgia
    RSVPreF3—respiratory syncytial virus (NCT03814590) [68]Injection site pain, fatigue, headache
    RTS,S/AS01—Malaria (NCT00866619) [69]Fever, irritability, drowsiness, loss of appetite
    M72/AS01—tuberculosis (NCT01755598) [70]Injection-site pain and influenza-like symptoms
    AS04MPL and alumCervarix®—papillomavirus (approved vaccine) [71]Injection site pain, redness and swelling, fatigue, gastrointestinal symptoms, headache
    Fendrix®—hepatitis B (approved vaccine) [72]Injection site pain, fatigue, headache, fever
    CpG 1018CpG oligodeoxynucleotidesHEPLISAV-B®—hepatitis B (approved vaccine) [73]Injection site pain, headache, fatigue
    Alum/OMVsAlum and OMVs from N. meningitidisSoberana 01—COVID-19 (RPCEC00000338) [65]Injection site pain and redness
    Display full size

    SUIV: supra-seasonal universal influenza vaccines

    Nonetheless, it is important to highlight that the adverse events might change according to each vaccine, when the antigen or the combination of adjuvants is considered. For example, most studies using AS01 reported mainly mild or moderate adverse events [67, 68], but a clinical trial using this adjuvant for a tuberculosis vaccines reported a case of pyrexia and two cases of immune-mediated disorders [70]. Moreover, a follow-up phase III study comprising approximately 15,000 patients found 16/5,949 and 9/4,358 cases of meningitis in children and infants, respectively, following vaccination with P. falciparum antigen and AS01 adjuvant [74], showing the relevance of conducting follow-up studies.


    This brief review summarizes important aspects related to immunization that were relevant in recent public health emergencies. Trained immunity conferred by BCG probably reduced the burden of COVID-19 in some populations. On the other hand, the immune response to vaccinia might have cross-reacted with monkeypox. These aspects were not enough to contain any of the outbreaks; however, they show the heterologous benefits of vaccines and support the claims for better vaccine coverage worldwide. Furthermore, novel adjuvant options and combinations should be investigated, aiming similar features for subunit vaccines.



    Adjuvant System


    Bacille Calmette-Guerin


    coronavirus disease 2019




    monophosphoryl lipid A


    outer membrane vesicles


    pathogen-associated molecular patterns


    pattern-recognition receptors


    severe acute respiratory syndrome coronavirus 2


    Author contribution

    AIP: Conceptualization, Investigation, Writing—original draft. EDG: Conceptualization, Investigation, Supervision, Writing—review & editing.

    Conflicts of interest

    The authors have no conflicts of interest regarding the publication of this article.

    Ethical approval

    Not applicable.

    Consent to participate

    Not applicable.

    Consent to publication

    Not applicable.

    Availability of data and materials

    Not applicable.


    This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), finance code [001], and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), grant number [18/04202-0]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.


    © The Author(s) 2023.


    World Health Organization. Health emergencies list [Internet]. c2023 [cited 2023 Feb 09]. Available from: https://www.who.int/emergencies/situations
    Li Q, Bergquist R, Grant L, Song JX, Feng XY, Zhou XN. Consideration of COVID-19 beyond the human-centred approach of prevention and control: the ONE-HEALTH perspective. Emerg Microbes Infect. 2022;11:25208. [DOI] [PubMed] [PMC]
    Murray KA, Daszak P. Human ecology in pathogenic landscapes: two hypotheses on how land use change drives viral emergence. Curr Opin Virol. 2013;3:7983. [DOI] [PubMed] [PMC]
    El-Sayed A, Kamel M. Climatic changes and their role in emergence and re-emergence of diseases. Environ Sci Pollut Res Int. 2020;27:2233652. [DOI] [PubMed] [PMC]
    Link-Gelles R, Lutterloh E, Schnabel Ruppert P, Backenson PB, St George K, Rosenberg ES, et al.; 2022 U.S. Poliovirus Response Team. Public health response to a case of paralytic poliomyelitis in an unvaccinated person and detection of poliovirus in wastewater — New York, June–August 2022. MMWR Morb Mortal Wkly Rep. 2022;71:10658. [DOI] [PubMed] [PMC]
    Mercader-Barceló J, Otu A, Townley TA, Adepoju P, Walley J, Okoibhole LO, et al. Rare recurrences of poliomyelitis in non-endemic countries after eradication: a call for global action. Lancet Microbe. 2022;3:e8912. [DOI] [PubMed]
    Paniz-Mondolfi AE, Tami A, Grillet ME, Márquez M, Hernández-Villena J, Escalona-Rodríguez MA, et al. Resurgence of vaccine-preventable diseases in Venezuela as a regional public health threat in the Americas. Emerg Infect Dis. 2019;25:62532. [DOI] [PubMed] [PMC]
    de Oliveira IS, Cardoso LS, Ferreira IG, Alexandre-Silva GM, Jacob BdCdS, Cerni FA, et al. Anti-vaccination movements in the world and in Brazil. Rev Soc Bras Med Trop. 2022;55:e0592-2021. [DOI] [PubMed] [PMC]
    Yang L, Grenfell BT, Mina MJ. Waning immunity and re-emergence of measles and mumps in the vaccine era. Curr Opin Virol. 2020;40:4854. [DOI] [PubMed]
    Huang YJS, Higgs S, Vanlandingham DL. Emergence and re-emergence of mosquito-borne arboviruses. Curr Opin Virol. 2019;34:1049. [DOI] [PubMed]
    Gonzalez-Perez M, Sanchez-Tarjuelo R, Shor B, Nistal-Villan E, Ochando J. The BCG vaccine for COVID-19: first verdict and future directions. Front Immunol. 2021;12:632478. [DOI] [PubMed] [PMC]
    Yitbarek K, Abraham G, Girma T, Tilahun T, Woldie M. The effect of Bacillus Calmette-Guérin (BCG) vaccination in preventing severe infectious respiratory diseases other than TB: implications for the COVID-19 pandemic. Vaccine. 2020;38:637480. [DOI] [PubMed] [PMC]
    Kulesza J, Kulesza E, Koziński P, Karpik W, Broncel M, Fol M. BCG and SARS-CoV-2—what have we learned? Vaccines (Basel). 2022;10:1641. [DOI] [PubMed] [PMC]
    Poland GA, Kennedy RB, Tosh PK. Prevention of monkeypox with vaccines: a rapid review. Lancet Infect Dis. 2022;22:e34958. [DOI] [PubMed] [PMC]
    Shafaati M, Zandi M. State-of-the-art on monkeypox virus: an emerging zoonotic disease. Infection. 2022;50:142530. [DOI] [PubMed]
    Saadatian-Elahi M, Aaby P, Shann F, Netea MG, Levy O, Louis J, et al. Heterologous vaccine effects. Vaccine. 2016;34:392330. [DOI] [PubMed]
    Netea MG, Domínguez-Andrés J, Barreiro LB, Chavakis T, Divangahi M, Fuchs E, et al. Defining trained immunity and its role in health and disease. Nat Rev Immunol. 2020;20:37588. [DOI] [PubMed] [PMC]
    Christensen JE, Thomsen AR. Co-ordinating innate and adaptive immunity to viral infection: mobility is the key. APMIS. 2009;117:33855. [DOI] [PubMed]
    Mulder WJM, Ochando J, Joosten LAB, Fayad ZA, Netea MG. Therapeutic targeting of trained immunity. Nat Rev Drug Discov. 2019;18:55366. [DOI] [PubMed] [PMC]
    Taks EJM, Moorlag SJCFM, Netea MG, van der Meer JWM. Shifting the immune memory paradigm: trained immunity in viral infections. Annu Rev Virol. 2022;9:46989. [DOI] [PubMed]
    Sørup S, Villumsen M, Ravn H, Benn CS, Sørensen TIA, Aaby P, et al. Smallpox vaccination and all-cause infectious disease hospitalization: a Danish register-based cohort study. Int J Epidemiol. 2011;40:95563. [DOI] [PubMed]
    Bottomley C, Bojang A, Smith PG, Darboe O, Antonio M, Foster-Nyarko E, et al. The impact of childhood vaccines on bacterial carriage in the nasopharynx: a longitudinal study. Emerg Themes Epidemiol. 2015;12:1. [DOI] [PubMed] [PMC]
    Sørup S, Benn CS, Poulsen A, Krause TG, Aaby P, Ravn H. Live vaccine against measles, mumps, and rubella and the risk of hospital admissions for nontargeted infections. JAMA. 2014;311:82635. [DOI] [PubMed]
    Lund N, Andersen A, Hansen ASK, Jepsen FS, Barbosa A, Biering-Sørensen S, et al. The effect of oral polio vaccine at birth on infant mortality: a randomized trial. Clin Infect Dis. 2015;61:150411. [DOI] [PubMed] [PMC]
    Düppre NC, Camacho LAB, da Cunha SS, Struchiner CJ, Sales AM, Nery JAC, et al. Effectiveness of BCG vaccination among leprosy contacts: a cohort study. Trans R Soc Trop Med Hyg. 2008;102:6318. [DOI] [PubMed]
    de Castro MJ, Pardo-Seco J, Martinón-Torres F. Nonspecific (heterologous) protection of neonatal BCG vaccination against hospitalization due to respiratory infection and sepsis. Clin Infect Dis. 2015;60:16119. [DOI] [PubMed]
    Biering-Sørensen S, Aaby P, Napirna BM, Roth A, Ravn H, Rodrigues A, et al. Small randomized trial among low-birth-weight children receiving bacillus Calmette-Guérin vaccination at first health center contact. Pediatr Infect Dis J. 2012;31:3068. [DOI] [PubMed]
    Arts RJW, Moorlag SJCFM, Novakovic B, Li Y, Wang SY, Oosting M, et al. BCG vaccination protects against experimental viral infection in humans through the induction of cytokines associated with trained immunity. Cell Host Microbe. 2018;23:89100.e5. [DOI] [PubMed]
    Escobar LE, Molina-Cruz A, Barillas-Mury C. BCG vaccine protection from severe coronavirus disease 2019 (COVID-19). Proc Natl Acad Sci U S A. 2020;117:177206. Erratum in: Proc Natl Acad Sci U S A. 2020;117:27741–2. [DOI] [PubMed] [PMC]
    Sato APS. Pandemic and vaccine coverage: challenges of returning to schools. Rev Saude Publica. 2020;54:115. [DOI] [PubMed] [PMC]
    Maltezou HC, Medic S, Cassimos DC, Effraimidou E, Poland GA. Decreasing routine vaccination rates in children in the COVID-19 era. Vaccine. 2022;40:25257. [DOI] [PubMed] [PMC]
    Pan American Health Organization. COVID-19 pandemic fuels largest continued backslide in vaccinations in three decades [Internet]. c2022 [cited 2023 Feb 01]. Available from: https://www.paho.org/en/news/15-7-2022-covid-19-pandemic-fuels-largest-continued-backslide-vaccinations-three-decades
    See KC. Vaccination for monkeypox virus infection in humans: a review of key considerations. Vaccines (Basel). 2022;10:1342. [DOI] [PubMed] [PMC]
    World Health Organization. Vaccines and immunization for monkeypox: interim guidance, 16 November 2022 [Internet]. c2023 [cited 2023 Feb 01]. Available from: http://www.who.int/publications/i/item/WHO-MPX-Immunization
    Ahmed SF, Sohail MS, Quadeer AA, McKay MR. Vaccinia-virus-based vaccines are expected to elicit highly cross-reactive immunity to the 2022 monkeypox virus. Viruses. 2022;14:1960. [DOI] [PubMed] [PMC]
    Gilchuk I, Gilchuk P, Sapparapu G, Lampley R, Singh V, Kose N, et al. Cross-neutralizing and protective human antibody specificities to poxvirus infections. Cell. 2016;167:68494.e9. [DOI] [PubMed] [PMC]
    Gu X, Zhang Y, Jiang W, Wang D, Lu J, Gu G, et al. Protective human anti-poxvirus monoclonal antibodies are generated from rare memory B cells isolated by multicolor antigen tetramers. Vaccines (Basel). 2022;10:1084. [DOI] [PubMed] [PMC]
    Gushchin VA, Ogarkova DA, Dolzhikova IV, Zubkova OV, Grigoriev IV, Pochtovyi AA, et al. Estimation of anti-orthopoxvirus immunity in Moscow residents and potential risks of spreading monkeypox virus. Front Immunol. 2022;13:1023164. [DOI] [PubMed] [PMC]
    Panchanathan V, Chaudhri G, Karupiah G. Protective immunity against secondary poxvirus infection is dependent on antibody but not on CD4 or CD8 T-cell function. J Virol. 2006;80:63338. [DOI] [PubMed] [PMC]
    Panchanathan V, Chaudhri G, Karupiah G. Correlates of protective immunity in poxvirus infection: where does antibody stand? Immunol Cell Biol. 2008;86:806. [DOI] [PubMed]
    Amara RR, Nigam P, Sharma S, Liu J, Bostik V. Long-lived poxvirus immunity, robust CD4 help, and better persistence of CD4 than CD8 T cells. J Virol. 2004;78:38116. [DOI] [PubMed] [PMC]
    Kennedy RB, Poland GA. The identification of HLA class II-restricted T cell epitopes to vaccinia virus membrane proteins. Virology. 2010;408:23240. [DOI] [PubMed] [PMC]
    Liu L, Zhong Q, Tian T, Dubin K, Athale SK, Kupper TS. Epidermal injury and infection during poxvirus immunization is crucial for the generation of highly protective T cell-mediated immunity. Nat Med. 2010;16:2247. [DOI] [PubMed] [PMC]
    Tscharke DC, Karupiah G, Zhou J, Palmore T, Irvine KR, Haeryfar SM, et al. Identification of poxvirus CD8+ T cell determinants to enable rational design and characterization of smallpox vaccines. J Exp Med. 2005;201:95104. [DOI] [PubMed] [PMC]
    Gillard GO, Bivas-Benita M, Hovav AH, Grandpre LE, Panas MW, Seaman MS, et al. Thy1+ NK cells from vaccinia virus-primed mice confer protection against vaccinia virus challenge in the absence of adaptive lymphocytes. PLoS Pathog. 2011;7:e1002141. Erratum in: PLoS Pathog. 2011;7. [DOI] [PubMed] [PMC]
    Arbel R, Sagy YW, Zucker R, Arieh NG, Markovits H, Abu-Ahmad W, et al. Vaccine effectiveness of modified vaccinia Ankara in human monkeypox. Res Sq [Preprint]. 2022 [cited 2023 Jan 30]. Available from: https://doi.org/10.21203/rs.3.rs-1976861/v1
    Fine PEM, Jezek Z, Grab B, Dixon H. The transmission potential of monkeypox virus in human populations. Int J Epidemiol. 1988;17:64350. [DOI] [PubMed]
    Luciani L, Lapidus N, Amroun A, Falchi A, Souksakhone C, Mayxay M, et al. Orthopoxvirus seroprevalence and infection susceptibility in France, Bolivia, Laos, and Mali. Emerg Infect Dis. 2022;28:246371. [DOI] [PubMed] [PMC]
    Hazra A, Rusie L, Hedberg T, Schneider JA. Human monkeypox virus infection in the immediate period after receiving modified vaccinia Ankara vaccine. JAMA. 2022;328:20647. [DOI] [PubMed] [PMC]
    Payne AB, Ray LC, Cole MM, Canning M, Houck K, Shah HJ, et al. Reduced risk for mpox after receipt of 1 or 2 doses of JYNNEOS vaccine compared with risk among unvaccinated persons — 43 U.S. Jurisdictions, July 31–October 1, 2022. MMWR Morb Mortal Wkly Rep. 2022;71:15604. [DOI] [PubMed] [PMC]
    Blasi F, Gramegna A, Sotgiu G, Saderi L, Voza A, Aliberti S, et al. SARS-CoV-2 vaccines: a critical perspective through efficacy data and barriers to herd immunity. Respir Med. 2021;180:106355. [DOI] [PubMed] [PMC]
    Moyle PM, Toth I. Modern subunit vaccines: development, components, and research opportunities. ChemMedChem. 2013;8:36076. [DOI] [PubMed]
    Grifoni A, Zhang Y, Tarke A, Sidney J, Rubiro P, Reina-Campos M, et al. Defining antigen targets to dissect vaccinia virus and monkeypox virus-specific T cell responses in humans. Cell Host Microbe. 2022;30:166270.e4. [DOI] [PubMed] [PMC]
    Gilchuk P, Hill TM, Guy C, McMaster SR, Boyd KL, Rabacal WA, et al. A distinct lung-interstitium-resident memory CD8+ T cell subset confers enhanced protection to lower respiratory tract infection. Cell Rep. 2016;16:18009. [DOI] [PubMed] [PMC]
    Heraud JM, Edghill-Smith Y, Ayala V, Kalisz I, Parrino J, Kalyanaraman VS, et al. Subunit recombinant vaccine protects against monkeypox. J Immunol. 2006;177:255264. [DOI] [PubMed]
    Goodridge HS, Ahmed SS, Curtis N, Kollmann TR, Levy O, Netea MG, et al. Harnessing the beneficial heterologous effects of vaccination. Nat Rev Immunol. 2016;16:392400. [DOI] [PubMed] [PMC]
    Miyaji EN, Carvalho E, Oliveira MLS, Raw I, Ho PL. Trends in adjuvant development for vaccines: DAMPs and PAMPs as potential new adjuvants. Braz J Med Biol Res. 2011;44:50013. [DOI] [PubMed]
    Munira SL, Hendriks JT, Atmosukarto II, Friede MH, Carter LM, Butler JRG, et al. A cost analysis of producing vaccines in developing countries. Vaccine. 2019;37:124551. [DOI] [PubMed]
    Luchner M, Reinke S, Milicic A. TLR agonists as vaccine adjuvants targeting cancer and infectious diseases. Pharmaceutics. 2021;13:142. [DOI] [PubMed] [PMC]
    Mekonnen D, Mengist HM, Jin T. SARS-CoV-2 subunit vaccine adjuvants and their signaling pathways. Expert Rev Vaccines. 2022;21:6981. [DOI] [PubMed] [PMC]
    Zhang N, Li K, Liu Z, Nandakumar KS, Jiang S. A perspective on the roles of adjuvants in developing highly potent COVID-19 vaccines. Viruses. 2022;14:387. [DOI] [PubMed] [PMC]
    Scaria PV, Rowe CG, Chen BB, Dickey TH, Renn JP, Lambert LE, et al. Protein-protein conjugation enhances the immunogenicity of SARS-CoV-2 receptor-binding domain (RBD) vaccines. iScience. 2022;25:104739. [DOI] [PubMed] [PMC]
    Jangra S, Landers JJ, Rathnasinghe R, O’Konek JJ, Janczak KW, Cascalho M, et al. A combination adjuvant for the induction of potent antiviral immune responses for a recombinant SARS-CoV-2 protein vaccine. Front Immunol. 2021;12:729189. [DOI] [PubMed] [PMC]
    Xiao Y, Zeng Y, Alexander E, Mehta S, Joshi SB, Buchman GW, et al. Adsorption of recombinant poxvirus L1-protein to aluminum hydroxide/CpG vaccine adjuvants enhances immune responses and protection of mice from vaccinia virus challenge. Vaccine. 2013;31:31926. [DOI] [PubMed] [PMC]
    Pérez-Rodríguez S, de la Caridad Rodríguez-González M, Ochoa-Azze R, Climent-Ruiz Y, Alberto González-Delgado C, Paredes-Moreno B, et al. A randomized, double-blind phase I clinical trial of two recombinant dimeric RBD COVID-19 vaccine candidates: safety, reactogenicity and immunogenicity. Vaccine. 2022;40:206875. [DOI] [PubMed] [PMC]
    Wheeler CM, Castellsagué X, Garland SM, Szarewski A, Paavonen J, Naud P, et al.; HPV PATRICIA Study Group. Cross-protective efficacy of HPV-16/18 AS04-adjuvanted vaccine against cervical infection and precancer caused by non-vaccine oncogenic HPV types: 4-year end-of-study analysis of the randomised, double-blind PATRICIA trial. Lancet Oncol. 2012;13:10010. Erratum in: Lancet Oncol. 2012;13:e1. [DOI] [PubMed]
    Folschweiller N, Vanden Abeele C, Chu L, Van Damme P, García-Sastre A, Krammer F, et al. Reactogenicity, safety, and immunogenicity of chimeric haemagglutinin influenza split-virion vaccines, adjuvanted with AS01 or AS03 or non-adjuvanted: a phase 1–2 randomised controlled trial. Lancet Infect Dis. 2022;22:106275. Erratum in: Lancet Infect Dis. 2022;22:e159. [DOI] [PubMed]
    Leroux-Roels I, Davis MG, Steenackers K, Essink B, Vandermeulen C, Fogarty C, et al. Safety and immunogenicity of a respiratory syncytial virus prefusion F (RSVPreF3) candidate vaccine in older adults: phase 1/2 randomized clinical trial. J Infect Dis. 2023;227:76172. [DOI] [PubMed] [PMC]
    RTS,S Clinical Trials Partnership; Agnandji ST, Lell B, Fernandes JF, Abossolo BP, Methogo BG, Kabwende AL, et al. A phase 3 trial of RTS,S/AS01 malaria vaccine in African infants. N Engl J Med. 2012;367:228495. [DOI] [PubMed]
    Tait DR, Hatherill M, Van Der Meeren O, Ginsberg AM, Van Brakel E, Salaun B, et al. Final analysis of a trial of M72/AS01E vaccine to prevent tuberculosis. N Engl J Med. 2019;381:242939. [DOI] [PubMed]
    Harper DM, Franco EL, Wheeler C, Ferris DG, Jenkins D, Schuind A, et al.; GlaxoSmithKline HPV Vaccine Study Group. Efficacy of a bivalent L1 virus-like particle vaccine in prevention of infection with human papillomavirus types 16 and 18 in young women: a randomised controlled trial. Lancet. 2004;364:175765. [DOI] [PubMed]
    Tong NKC, Beran J, Kee SA, Miguel JL, Sánchez C, Bayas JM, et al. Immunogenicity and safety of an adjuvanted hepatitis B vaccine in pre-hemodialysis and hemodialysis patients. Kidney Int. 2005;68:2298303. [DOI] [PubMed]
    Halperin SA, Ward B, Cooper C, Predy G, Diaz-Mitoma F, Dionne M, et al. Comparison of safety and immunogenicity of two doses of investigational hepatitis B virus surface antigen co-administered with an immunostimulatory phosphorothioate oligodeoxyribonucleotide and three doses of a licensed hepatitis B vaccine in healthy adults 18–55 years of age. Vaccine. 2012;30:255663. [DOI] [PubMed]
    RTS,S Clinical Trials Partnership. Efficacy and safety of the RTS,S/AS01 malaria vaccine during 18 months after vaccination: a phase 3 randomized, controlled trial in children and young infants at 11 African sites. PLoS Med. 2014;11:e1001685. [DOI] [PubMed] [PMC]