Vaccine effectiveness of Pfizer-BioNTech and Oxford-AstraZeneca to prevent severe COVID-19 in Costa Rica by September and October 2021: A nationwide, observational study of hospitalisations prevalence

Objective To estimate the dose-dependent effectiveness of coronavirus disease (COVID-19) vaccines to prevent severe illness in real-world conditions of Costa Rica, after the Delta variant became dominant. Design Observational study; secondary analysis of hospitalisation prevalence. Setting Nationwide adult population, Costa Rica. Participants All 3.67 million adults residents in Costa Rica by mid-2021. Public aggregated data of 5978 hospital records from 14th September to 20th October, 2021 and 6.1 million vaccination doses administered. Interventions Vaccination with Pfizer-BioNTech (78%) and Oxford-AstraZeneca (22%). Main outcome measures Prevalence of COVID-19-related hospitalisations Results Vaccine effectiveness to prevent hospitalisation (VEH) was estimated as 93.4% (95% confidence interval [CI]: 93.0 to 93.9) for complete vaccination and 76.7% (CI: 75.0 to 78.3) for single-dose vaccination among adults of all ages. VEH was lower and more uncertain among older adults aged 58 years and above: 92% (CI: 91% to 93%) for those who had received full vaccination and 64% (CI: 58% to 69%) for those who had received partial vaccination. Single-dose VEH declined over time during the study period, especially in the older age group. Estimates were sensitive to possible errors in the population count used to determine the residual number of unvaccinated people in groups with high vaccine coverage. Conclusion The Costa Rican vaccination programme that administered Pfizer and Oxford vaccines are highly effective to prevent COVID-19-related hospitalisations after the Delta variant had become dominant. Moreover, a single dose is reasonably effective, justifying the continuation of the national policy of postponing the application for the second dose of the Pfizer vaccine to accelerate the vaccination and increase the number of people being vaccinated. Timely monitoring of vaccine effectiveness is important to detect eventual failures and motivate the public based on information that the vaccinations are effective. Summary Box What is already known on the topic The Costa Rican Social Security Fund provides vaccinations in Costa Rica, and they use Pfizer-BioNTech and Oxford-AstraZeneca for the vaccinations. The Delta variant became predominant in Costa Rica by September 2021 Real-world estimates of these vaccines effectiveness to prevent hospitalisations range from 90% to 98% for two doses and from 70% to 91% for a single dose. Almost all of these estimates predate the Delta variant. There are controversies regarding the effectiveness of a single dose of COVID-19 vaccine after the emergence of the Delta variant. What this study adds Vaccine effectiveness to prevent hospitalisation as estimated as 93.4% (95% CI: 93.0 to 93.9) for complete vaccination and 76.7% (CI: 75.0 to 78.3) for single-dose vaccination among adults of all ages by October 2021. These study findings suggest that a single dose of COVID-19 vaccination is reasonably effective to prevent hospitalisations. Therefore, the application for the second dose of the Pfizer vaccine can be postponed beyond the three weeks recommended by the fabricant to accelerate vaccination coverage.