JMIR Publications

ABSTRACT

Background:

The Costa Rican vaccination program uses Pfizer-BioNTech and Oxford-AstraZeneca vaccines. Real-world estimates of these vaccines effectiveness to prevent hospitalizations 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.

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.

Methods:

This observational study is a secondary analysis of hospitalizations prevalence. The participants are all 3.67 million adults residents in Costa Rica by mid-2021. The study is based on public aggregated data of 5978 COVID-19-related hospital records from 14th September to 20th October, 2021 and 6.1 million vaccination doses administered to determine hospitalization prevalence by dose-specific vaccination status. The intervention retrospectively evaluated is vaccination with Pfizer-BioNTech (78%) and Oxford-AstraZeneca (22%). The main outcome studied is being hospitalized.

Results:

Vaccine effectiveness to prevent hospitalization (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 when vaccine coverage is high.

Conclusions:

The Costa Rican vaccination program that administered Pfizer and Oxford vaccines are highly effective to prevent COVID-19-related hospitalizations 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.