Estimating vaccine effectiveness against SARS-CoV-2 infection, hospitalization and death from ecologic data in Costa Rica

Abstract

Background: Clinical trials and individual-level observational data in Israel demonstrated approximately 95% effectiveness of mRNA-based vaccines against symptomatic SARS-CoV-2 infection. Individual-level data are not available in many countries, particularly low- and middle- income countries. Using a novel Poisson regression model, we analyzed ecologic data in Costa Rica to estimate vaccine effectiveness and assess the usefulness of this approach.

Methods: We used national data from December 1, 2020 to May 13, 2021 to ascertain incidence, hospitalizations and deaths within ecologic units defined by 14 age groups, gender, 105 geographic areas, and day of the epidemic. Within each unit we used the proportions of the population with one and with two vaccinations, primarily tozinameran. Using a non-standard Poisson regression model that included an ecologic-unit-specific rate factor to describe rates without vaccination and a factor that depended on vaccine effectiveness parameters and proportions vaccinated, we estimated vaccine effectiveness.

Results: In 3.621 million persons aged 20 or older, there were 125,031 incident cases, 7716 hospitalizations, and 1929 deaths following SARS-CoV-2 diagnosis; 73% of those aged ≥ 75 years received two doses. For one dose, estimated effectiveness was 59% (95% confidence interval 53% to 64%) for SARS-CoV-2 incidence, 76% (68% to 85%) for hospitalizations, and 63% (47% to 80%) for deaths. For two doses, the respective estimates of effectiveness were 93% (90% to 96%), 100% (97% to 100%), and 100% (97% to 100%).

Conclusions: These effectiveness estimates agree well with findings from clinical trials and individual-level observational studies and indicate high effectiveness in the general population of Costa Rica. This novel statistical approach is promising for countries where ecologic, but not individual-level, data are available. The method could also be adapted to monitor vaccine effectiveness over calendar time.

Keywords: COVID-19; Ecologic estimate of vaccine effectiveness; SARS-CoV-2; Vaccine effectiveness.

Fig. 1

Percentage of people vaccinated with 1 and with 2 doses by age group plotted against calendar time (January 1, 2021 to May 13, 2021). A person is defined as vaccinated with two doses on a given day if the second dose was received at least 14 days previously. A person is defined as vaccinated with one dose on a given day if the first vaccination was received at least 14 days previously and no second dose was given more than 14 days previously. Black, dark gray and light gray lines correspond respectively to 20–59, 60–74 and ≥ 75-year age groups. Solid lines are for one dose and dashed lines for two doses

Fig. 2

Semi-logarithmic plot of COVID-19 incidence (A), hospitalizations (B), and deaths (C) per 10⁵ people against calendar time (December 7, 2020 to May 13, 2021) by age group. Incidence, hospitalizations, and deaths were estimated from the preceding seven-day moving average. Black, dark grey and light grey lines correspond respectively to 20–59, 60–74 and ≥ 75-year age groups

Fig. 3

Plots of predicted numbers of COVID-19 deaths against day of epidemic without vaccination (black solid line) and with vaccination (black dashed line) respectively for persons aged 20–59 (A), 60–74 (B), and 75 + (C) years. The area between these plots represents the estimated number of deaths prevented by vaccination. Also shown are the observed numbers of deaths (seven-day average, grey line), which agree well with the dashed line that accounts for vaccination