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Microbes & Immunity Homologous versus heterologous COVID-19 vaccines

1. Introduction heterologous vaccinations, which involve administering
a prime and booster vaccine developed from different
On December 31, 2019, several hospitals in Wuhan, platforms, has been widely adopted in many countries.
China, reported increasing clusters of patients suffering According to interim guidance from the WHO, the
from acute atypical pneumonia of unknown etiology. rationale for implementing heterologous vaccines was
The causative pathogen was later identified to be a novel due to concerns of inadequate vaccine supply and delay of
coronavirus that was named severe acute respiratory the administration of booster doses, especially in lower-
syndrome coronavirus-2 (SARS-CoV-2). The World Health income countries. More importantly, heterologous vaccines
Organization (WHO) termed the following respiratory confer better immunity against COVID-19 and enhance
illness as coronavirus disease 2019 (COVID-19) and vaccine efficacy. Therefore, the objective of this paper
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subsequently declared it a global pandemic in March 2020 is to conduct a scoping review of the existing scientific
due to the outbreak of the virus that affected approximately evidence to compare the efficacy of homologous and
200 countries worldwide. As of May 2024, data from WHO heterologous vaccination strategies, as well as demonstrate
indicate a cumulative number of 775 million confirmed that heterologous vaccinations are more efficacious and
cases of COVID-19, with 7 million deaths. 1 confer better immunity against COVID-19.
The SARS-CoV-2 is a coronavirus containing positive-
stranded RNA. Its spike proteins, located on its surface, 2. Methodology
bind to angiotensin-converting enzyme 2 through receptor- This scoping review was conducted in accordance with
binding domains (RBD) in these proteins, facilitating entry Arksey and O’Malley’s five-stage framework.
into host cells. The virus spreads through human-human
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transmission, primarily when mucosal surfaces (nose, Arksey, H. and O’Malley, L., 2005. Scoping studies:
mouth, and eyes) are exposed to air droplets or fomites toward a methodological framework. International journal
containing the virus. The severity of infection can range of social research methodology, 8(1), pp.19-32.
from mild to moderate to severe, with common symptoms 2.1. Research question
including fever, cough, shortness of breath, headache,
lethargy, anosmia, and ageusia. 2 The research question for the study was, “Do heterologous
vaccines provide better immunity against COVID-19 when
The COVID-19 pandemic is undeniably a major public compared to homologous vaccines?” The PICO framework
health crisis, posing significant threats to both individual was used to define the research question as well as develop
health and global healthcare systems and economies. Over alternate terms for the literature search (Table 1).
the past 5 years, various strategies have been implemented
by governments and health authorities to mitigate the 2.2. Identification of relevant studies
spread of the virus, including social distancing, mandatory Three databases (Ovid MEDLINE, PubMed, and Scopus)
mask-wearing, quarantine of infected individuals and their were used to source relevant studies. During the literature
close contacts, widespread lockdowns, and international search, comprehensive keywords including “heterologous
travel bans. In addition, numerous companies have vaccines,” “homologous vaccines,” “COVID-19,” “SARS-
invested heavily in vaccine development since the start CoV-2,” and “immunogenicity” were employed, along with
of the pandemic, driven by the strong belief that vaccines MeSh terms and Boolean operators (“OR” and “AND”)
are preventative medicine that can effectively reduce the to combine keywords and refine the search terms. Given
rate of infections and decrease morbidity and mortality the extensive research on COVID-19, the search initially
from COVID-19. Various vaccines have been approved by yielded an extensive number of articles. To narrow the
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the WHO for inclusion in global COVID-19 vaccination results, the search was limited to studies published from
programs or for emergency use, with their efficacy and 2020 – 2024 and in English, focusing on clinical trials or
safety profiles well-established through evidence from observational studies.
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extensive clinical trials. These vaccines include Pfizer/
BioNTech (BNT 162b2), Moderna (mRNA-1273), Table 1. PICO template used to develop search terms
Oxford/AstraZeneca (ChAdOx1-S), Johnson and Johnson PICO terms Description
(Ad 26.COV2-S), Sinopharm (BBIP-CorV), Sinovac
(CoronaVac), Covaxin (BBV152), and Novavax (NVX- P: Patient/Problem COVID-19
CoV2373). 4 I: Intervention Heterologous vaccination
Although homologous vaccinations remain C: Comparison Homologous vaccination
the traditional approach of vaccination, the use of O: Outcome Stronger anti-vaccine immune response

Volume 1 Issue 2 (2024) 13 doi: 10.36922/mi.3757