https://orcid.org/0000-0003-1396-4928
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ABSTRACT
Background
The waning of the protective effect of COVID-19 vaccines and timing of booster doses are debated.
Methods
Population-based cohort study in the largest Health-Authority of Lazio region, Italy, on 946,156 residents aged 12+ (study period: 1 January 2021–10 January 2022). Vaccine effectiveness (VE) against any SARS-CoV-2 infection (symptomatic or asymptomatic) was estimated through multivariable negative-binomial models using unvaccinated person-time as a reference.
Results
The primary vaccination cycle was completed by 81% of residents; of these, 45% received a booster dose. Vaccine coverages were lower for foreigners, and people living in deprived areas, families with children aged 0–11, and households size 1 or 6+. Overall, VE waned from 71% (95% Confidence Interval (CI) 70–73%) 1 month after the second dose to 43% (CI 41–45%) after 4 months and 24% (CI 21–27%) after 6 months, especially in the elderly aged 70+. We observed a prompt restore of VE 15–19 days after the booster dose (69%, CI 67–70%).
Conclusions
Our results support the recommendation of a booster dose 4 months after completion of the primary cycle, giving priority to elderly and fragile individuals. The lower vaccine coverage among social disadvantaged subgroups suggests the need of targeted communication and interventions.
Author contributions statement
VF conceived the study. VF, EC, DC, and MF contributed to the design of the study. VF and AA performed the data preparation. VF and EC performed the statistical analyses. VF, EC, DC, and MF wrote the first draft of the manuscript. All authors: critically revised the manuscript and approved its final version; agreed on the journal to which the article will be submitted; reviewed and agreed on all versions of the article before submission, during revision, the final version accepted for publication, and any significant changes introduced at the proofing stage; agree to take responsibility and be accountable for the contents of the article and to share responsibility to resolve any questions raised about the accuracy or integrity of the published work.
Acknowledgments
The authors express their deepest gratitude to all healthcare workers engaged in the mass vaccination campaign, working hard, and in synergy with the personnel committed in the contact tracing and testing facilities. Great appreciation goes to the administrative personnel and volunteers welcoming the population at the vaccine hubs: with their invaluable assistance, it was possible to spread hope and resilience throughout the emergency crisis.
Ethical statement
This study, based on routinely collected data, was not submitted for approval to an ethical committee because the dissemination of COVID-19 surveillance data was authorized by the Italian Presidency of the Council of Ministers on 27 February 2020 (Ordinance no. 640). Because of the retrospective design and the large size of the population under study, in accordance with the Authorization n. 9 released by the Italian data protection authority on the 15th of December 2016, the individual informed consent was not requested for the conduction of this study.
Declaration of interest
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
Reviewer disclosures
Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.
Supplementary material
Supplemental data for this article can be accessed here