Evidence before this study
Vaccination is crucial for the control of COVID-19 and to date several vaccines have been deployed. It is important to assess and compare the real-life immunogenicity and effectiveness of the licensed vaccines. We searched PubMed and medRxiv for articles published up to July 14, 2021, using the search terms (“COVID-19” OR “SARS-CoV-2” OR “2019-nCoV”) AND (“vaccine” OR “vaccination”) AND (“immunogenicity” OR “efficacy” OR “effectiveness”); we then repeated the search by replacing the (“immunogenecity” OR “efficacy” OR “effectiveness) with “rollout” in an attempt to identify more studies. Our search yielded two studies of vaccine effectiveness, including one from Israel that assessed the effectiveness of Pfizer–BioNTech’s mRNA BNT162b2 vaccine against symptomatic infection and one study from Scotland that compared the effectiveness of BNT162b2 with that of Oxford–AstraZeneca’s ChAdOx1 nCoV-19. In this study from Scotland, a single dose of the BNT162b2 vaccine was 91% effective (95% CI 85–94) and of the ChAdOx1 vaccine was 88% effective (95% CI 75–94) in reducing COVID-19 hospital admissions 28–34 days after vaccination. The combined vaccine effect against hospital admission due to COVID-19 was 83% (95% CI 72–89) for people aged 80 years and older at 28–34 days after vaccination. For inactivated vaccines, we found no studies comparing immunogenicity, one underpowered study assessing the effectiveness of Sinovac’s CoronaVac in health-care workers, and one non-peer-reviewed study in older individuals done in a setting with substantial SARS-CoV-2 gamma (P.1) variant circulation, which showed a decline in Coronavac vaccine effectiveness against RT-PCR confirmed SARS-CoV-2 infection from 62% (95% CI 35–78) in people aged 70–74 years to 28% (0·6–48) in people aged 80 years and older. Population-based studies simultaneously assessing the performance of two or more vaccines are scarce.
Added value of this study
To the best of our knowledge, this is the only study to date to compare population IgG immunity after vaccination with CoronaVac (Sinovac Life Sciences, Beijing, China) and BNT162b2 vaccine in nearly 60 000 individuals in a country with rapid vaccine rollout. 4 weeks after one dose, the IgG seropositivity among CoronaVac recipients was 28·1% (95% CI 25·0–31·2) and among BNT162b2 recipients was 79·4% (75·7–83·1%). 3 weeks after the second vaccine dose, the IgG positivity was 77·4% (75·5–79·3) for the CoronaVac vaccine and 96·5% (94·9–98·1) for the BNT162b2 vaccine. A steady decrease in IgG seropositivity was observed in CoronaVac recipients 4–16 weeks after the second dose, whereas seropositivity remained high and stable in people who had received the BNT162b2 vaccine. IgG seropositivity was significantly lower in participants aged 60 years and older than in adults aged 18–39 years for both vaccines and significantly lower in men than women for the CoronaVac vaccine.
Implications of all the available evidence
Overall, IgG positivity for CoronaVac recipients reached 77% after two doses, and a single dose led to low IgG positivity levels (ie, seropositivity of 28%). Seropositivity in BNT162b2 vaccine recipients surpassed 95% after two doses and 80% after one vaccine dose. A steady decline in IgG seropositivity was observed for the CoronaVac vaccine from 4 weeks after full vaccination; however, this effect was not observed in people who had the BNT162b2 vaccine. Prolonged IgG monitoring will enable further evaluation of seropositivity over time, providing data in conjunction with effectiveness studies, for possible future reassessment of vaccination strategies.