It is worthy to note that Alagoz et al.
[68][36] hypothesized that if there is a strong adherence to non-pharmacological interventions in the community, the controllable spread of SARS-CoV-2 can be reached sooner than when a substantial part of the population gets vaccinated (e.g., 70–80%). In the current study, COVID-19 vaccines effectively reduced the incidence of symptomatic and asymptomatic infection. On the same lines, the WHO reported that unvaccinated persons account for the great majority of the current SARS-CoV-2 infection
[65][33]. Virus-neutralizing antibodies are principally responsible for the protection provided by presently available vaccinations. These antibodies often inhibit the virus’s binding with its cellular receptor or prevent the virus from undergoing the conformational changes essential for fusion with the cell membrane
[69][37]. WeIt's found that vaccination against COVID-19 decreased the number of cases reported within a week of the 2nd dose (OR = 0.06 (95% CI, 0.02–0.21), I
2 = 98%). Type of vaccine and country where study was conducted were the main predictors of vaccine efficacy and effectiveness. Similarly, the total number of cases diagnosed within 14 days of the 2nd dose decreased significantly, (OR = 0.01 [95% CI, 0.01–0.02], I
2 = 0%). In terms of cases reported 7 days after 2nd dose, the total number of cases decreased significantly with vaccination (OR = 0.03 [95% CI, 0.02–0.05], I
2 = 73%). About 100% of this heterogeneity was explained by meta-regression (vaccine type and country). Regarding symptomatic cases diagnosed 7 days after the 2nd dose, COVID-19 vaccine was effective in reducing the number of symptomatic cases in comparison to placebo or control group (OR = 0.02 [95% CI, 0.02–0.02], I
2 = 0%). The odds ratio of cases reported 14 days after the 2nd dose among vaccinated versus unvaccinated subjects was OR = 0.08, [95% CI, 0.02-0.34], I
2 = 100%). Confirmed cases reported after the 1st and 2nd dose regardless of the duration decreased significantly, OR = 0.14 (95% CI, 0.07–0.4) I
2 = 100% and 0.18 (95% CI, 0.15–0.19), I
2 = 98%, respectively.
In the same vein, many reviews addressed vaccine effectiveness and efficacy. Pormohammad et al.
[70][38] included 25 studies in phase II/III RCTs, the efficacy of mRNA-based and adenovirus-vectored COVID-19 vaccines was 94.6% and 80.2%, respectively. After 3 weeks of vaccinations, the adenovirus-vectored vaccine had the maximum efficacy against receptor-binding domain (RBD) antigen after the 1st and 2nd doses (97.6% and 98.2% respectively). Similarly, a review of phase III studies showed a significant increase in neutralizing antibodies with the 2nd dose of the vaccine
[71][39]. However, it was also advised that when vaccine supply is scarce, countries should vaccinate with a single dose. This may provide better overall protection in the population than vaccinating half the number of individuals with both doses
[72][40].
Many factors can explain the observed difference in efficacy and effectiveness of the COVID-19 vaccines. The Center for Disease Control and Prevention
[73][41] demonstrated that in the real-world, vaccine effectiveness can be affected by several factors, including population host factors (e.g., those who were not included in clinical trials) and virus factors (e.g., variants) as well as programmatic factors (e.g., adherence to dosing schedules or vaccine storage/handling)
[74][42]. Thompson et al.
[75][43] reported that under real-world conditions, complete immunization (14 days after 2nd dose) was 90% effective against SARS-CoV-2 infection, while partial immunization (14 days after 1st dose but before 2nd dose) was 80% effective. In addition, the effectiveness of vaccination varied according to the types of vaccine; Pilishvili et al.
[76][44] stated that vaccine effectiveness for Pfizer–BioNTech and Moderna were 77.6% (95.6% CI, 70.9–82.7) and 88.9% (95.9% CI, 78.7–94.2) after the 1st dose and were 88.8% (95% CI, 84.6–91.8) and 96.3% (95.3–98.4) after the 2nd dose, respectively. Of note, when the SARS-CoV-2 Delta variant became prevalent, the percentage of completely vaccinated people who got SARS-CoV-2 infection grew higher than predicted
[63][31]. The effectiveness of the mRNA vaccine against COVID-19 was 88–100% against Alpha, 76–100% against Beta/Gamma, 47.3–88% against Delta, and 89–100% when the SARS-CoV-2 strain was not sequenced. Oxford/AstraZeneca (AZD1222) was 74.5% effective against Alpha and 67% effective against Delta. CoronaVac was effective against the Alpha/Gamma/D614G strain in 36.8–73.8% of cases
[64][32].
Unfortunately, new data consistently demonstrated that vaccine efficacy against SARS-CoV-2 infection declines with time following immunization
[77][45]. It is worth noting that according to a recently published systematic review and meta-analysis, immunization efficacy against severe COVID-19 infection dropped by around 8% (95% CI, 4–15) during the 6-months period in all age groups. Over the same time, vaccine efficacy against serious illness declined by around 10% (95% CI, 6–15%) in individuals over the age of 50. Vaccine efficacy against symptomatic illness fell by 32% (95% CI, 11–69%) in individuals over the age of 50
[78][46]. Consequently, WHO has already recommended administering a booster dose of vaccine to people aged 60 years or older as part of the main series to strengthen initial protection
[65][33]. Therefore, people should adhere to public health and social measures even though they have received vaccines to avoid COVID-19 infection and its consequences
[79][47].