The outbreak of coronavirus disease in 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to a sharp increase in deaths and the number of infections worldwide. The pandemic has killed more than 5.1 million people from more than 254 million infections as of November 18, 2021. The development of safe and effective vaccines is therefore considered an important measure to control the pandemic and restore people’s lives to normal.
According to global statistics from August 2, 2021, there are a total of 326 vaccine candidates. Of these, 103 vaccines are in clinical trials and 19 are already in daily use. The 19 vaccines included 8 inactivated vaccines, 5 protein subunits, 2 mRNA vaccines, and 4 non-replicating viral vector vaccines. In addition, reports show that 53.7% of the world’s population has received at least one dose of the vaccine, while 41.5% have been fully vaccinated.
However, the rate of COVID-19 infection is still high due to the emergence of new variants of SARS-CoV-2. Therefore, rapid immunity of the herd through vaccination is needed to prevent the emergence of these new variants, which can completely escape immune surveillance.
The efficacy and safety of the three major vaccines on the market have been evaluated on the basis of randomized clinical trials (RCTs). MRNA vaccines have been found to be most effective, followed by viral vector vaccines and inactivated viral vaccines. Although the current safety of COVID-19 vaccines is high, long-term monitoring should be performed, especially for people with comorbidities. However, real-world studies differ significantly from RCTs. Mass vaccination in the real world requires consideration of several heterogeneous populations, supply of vaccines, desire, medical availability, etc. Several studies have reported the effectiveness of vaccines in the real world, but the results remain conflicting.
A new study published in Infectious diseases of poverty aims to systematically evaluate the efficacy and safety of COVID-19 vaccines in the real world, as well as to establish a reliable evidence-based basis to determine the true protective effect of COVID-19 vaccines, especially in view of the new waves of infection caused by the variants.
The study included a search in three databases, PubMed, Embase and Web of Science, for eligible studies published by July 22, 2021. It included 58 studies, of which 32 were for vaccine efficacy and 26 were for safety studies. The study was conducted strictly in accordance with the Preferred Reporting Elements for Systematic Reviews and Meta-Analysis (PRISMA).
Observational studies examining the efficacy and safety of COVID-19 vaccines in vaccinated individuals were included. Studies with inappropriate topics, insufficient data, overlapping participants, reviews, editorials, case reports, conference reports, animal experiments, and those that did not clarify the identification of COVID-19 were excluded.
The studies were identified by two researchers who performed data extraction and also assessed the quality of the study. A third investigator resolves any disagreement. Finally, a meta-analysis was performed with the derived data to evaluate the safety and efficacy of COVID-19 vaccines.
Block diagram of the study selection
The results of the study reported the effectiveness of the vaccine (VE) separately for the first and second dose. For the first dose of VE for infection prevention is 41 percent, for prevention of symptomatic infection with COVID-19 is 52 percent, for prevention of hospitalization is 66 percent, for prevention of admission to the intensive care unit is 45 percent, and for prevention of COVID – 19 related deaths were 53 percent.
While the second dose of VE for the prevention of SARS-CoV-2 infection was 85 percent, for the prevention of symptomatic COVID-19 was 97 percent, for the prevention of hospitalization was 93 percent, for the prevention of intensive care was 96 percent, and for COVID-19-related death prevention was 95 percent.
In addition, the study reported VE for different variants of SARS-CoV-2. VE was 85 percent for prevention against the Alpha variant, 54 percent for the Gamma variant, and 74 percent for the Delta variant. There was only one beta study where VE was reported to be 75 percent. The BNT162b2 vaccine was found to have the highest VE for each of the variants.
The study also reported the frequency of adverse events to determine the safety of COVID-19 vaccines. The incidence was found to be 1.5% for adverse events, 0.4% for serious adverse events, and 0.1% for death after vaccination.
Healthcare professionals show a higher incidence of adverse events, serious adverse events, and death after vaccination. It was also found that the incidence of adverse events was higher in the AZD1222 vaccine group, serious adverse events were higher in the Gam-COVID-Vac vaccine group, and post-vaccination death was higher in the BNT162b2 vaccine group.
Thus, the present study showed that two doses of the vaccine were effective against the different variants of SARS-CoV-2, although their effectiveness was lower compared to the original strain. Adverse reactions were found to vary between different groups of vaccines against COVID-19. However, serious adverse events have been reported, suggesting the need for a single global monitoring system to monitor adverse events from COVID-19 vaccines worldwide. In addition, the socio-economic characteristics of each country must be taken into account when prioritizing vaccines.
It can therefore be concluded that improving vaccine coverage is still the most important tool that can help end the pandemic.
The current study had certain limitations. First, the study includes limited baseline data on specific populations. Second, no subgroup analysis was performed for more population characteristics, such as age. Third, most of the initial studies collected adverse events only up to 7 days after vaccination. This may affect the safety analysis.