COVID-19 vaccines efficacy in preventing or limiting SARS-CoV-2 infections.

We read with interest the recent article of Hsu and colleagues, who concluded that vaccination against coronavirus disease 2019 (COVID-19) is highly effective to lower the risk of developing severe and/or life-threatening illness, but does not seem very efficient for averting the likelihood of becoming infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and, therefore, for preventing or limiting virus spread.

In order to provide further insights on this pivotal healthcare aspect, we used data from the nationwide COVID-19 vaccination campaign to assess to which extent COVID-19 vaccines may be effective for preventing newly diagnosed SARS-CoV-2 infections in the general Italian population. The information for this analysis was retrieved by accessing the report published on weekly basis by the Italian National Institute of Health (Istituto Superiore di Sanità, ISS; Last available update, January 4, 2022), and which contains official data on COVID-19 vaccinations and newly diagnosed cases of SARS-CoV-2 infection. The odds ratio (OR) with 95% confidence interval (95%CI) of SARS-CoV-2 infection in different cohorts was calculated with MetaXL, software Version 5.3 (EpiGear International Pty Ltd., Sunrise Beach, Australia). The study was carried out in accordance with Helsinki Declaration, under terms of relevant local legislation. The research was based on public ISS data, so that Ethical Committee approval was unnecessary.

The total number of people who completed a primary COVID-19 vaccination cycle within 120 days, by over 120 days, as well as of those who received a recent COVID-19 vaccine booster dose was 26.3, 13.6 and 5.7 million people at the time of this analysis (i.e., January 4, 2022), while 6.9 million people were still unvaccinated. Overall, the rate of SARS-CoV-2 infections per 10,000 was 248.0 (170,551/6,876,688) in unvaccinated people, decreasing to 100.9 (265,724/26,348,254), 73.4 (99,757/13,585,896) and 35.8 (20,375/5,694,939) in those who completed the primary COVID-19 vaccination by over 120 days, within 120 days and in those who received the booster dose, respectively. Compared to unvaccinated people these figures translated into a cumulatively lower risk of SARS-CoV-2 infection of 68% (OR, 0.32; 95%CI, 0.19–0.56) in the entire cohort of COVID-19 vaccine recipients, with such risk reduction being the highest in those who received the vaccine booster dose (OR, 0.14; 95%CI, 0.14–0.14), followed by those who completed the primary vaccination within 120 days (OR, 0.29; 95%CI, 0.29–0.29) and, finally, by those who had completed the primary vaccination cycle by over 120 days (OR, 0.40; 95%CI, 0.40–0.40) (Fig. 1 ).

Fig. 1

Impact of coronavirus disease (COVID-19) vaccination in preventing SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) infections in the general Italian population. Risk of infection compared to unvaccinated people.

OR, odds ratio; 95%CI, 95% confidence interval.

The results of our analysis on available data of the ongoing nationwide Italian COVID-19 vaccination campaign suggest that although the viral load may be basically similar between unvaccinated subjects with primary SARS-CoV-2 infection and vaccinated people with breakthrough infections, the overall risk of being infected by SARS-CoV-2 is nearly 70% lower in vaccine recipients, further decreasing to nearly 90% in those who received a vaccine booster dose after completing the primary vaccination cycle. According to our analysis, the risk that any vaccinated individual may become a virus spreader, as emphasized by Hsu et al., was hence found to be many times lower compared to unvaccinated people. This would lead us to conclude that COVID-19 vaccination should be further encouraged and supported, especially in countries with low vaccination rate, since this seems a quite reasonable and effective strategy for preventing or limiting SARS-CoV-2 circulation, thus ultimately reducing the medical, social and economic burden of SARS-CoV-2, as well as for mitigating the risk that new and highly mutated variants (like Omicron B.1.1.529) will emerge.

Funding

None declared.

Declaration of Competing Interest

The authors have no relevant competing interest to disclose in relation to this work.