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Using Serological Studies to Assess COVID-19 Infection Fatality Rate in Developing Countries: A Case Study from One Colombian Department

Our take —

This study measured IgM, IgG, and IgA antibodies in blood samples from randomly selected households across the Cordoba department in Colombia and calculated an estimated overall SARS-CoV-2 seroprevalence of 41% during the Fall of 2020 — ranging from 36% in people ages 50-59 years to 56% in adolescents ages 10-14 years. The infection fatality ratio among age-stratified groups was highest among people ages 70+ years (1.5%) and lowest for adolescents ages 10-14 years (0.001%). The overall case fatality ratio was 5.6% — ranging for 26.5% in people ages 70+ years to 0.1% among adolescents. The study time period used to estimate seroprevalence did not span the overall pandemic period from which they drew their mortality data, which may have biased these estimates. However, these data point to a high burden of infection in this population, across all age groups. This evidence also points to a low risk of mortality but frequent risk of infection among children.

Study design

Prospective Cohort

Study population and setting

The study sought to describe the infection fatality rate and estimate seroprevalence of SARS-CoV-2 in a department in Colombia. The Cordoba department in Colombia was selected for this study, spanning 6 cities. All individuals >9 years of age at the time were considered eligible, and households were randomly selected from the catchment area. All COVID-19 deaths in these cities reported between March 2020 and October 9th 2020, were included in the analysis. Seroprevalence was estimated by blood samples for IgM, IgG, and IgA antibodies using dual recognition ELISA, collected between August and September 2020. The case-fatality rate (CFR) and infection-fatality rate (IFR) were calculated using the reported deaths, reported detected COVID-19 cases, and population size by age-group. IFR was the number of COVID-19 deaths divided by the proportion of seropositive infections from the study, and the CFR was calculated by the number of COVID-19 deaths divided by the proportion of reported COVID-19 cases.

Summary of Main Findings

The study found that there were 1,618 COVID-19-related deaths in the department during the study period. There were a reported 661,963 cases of COVID-19 among a population of 1,554,788 people. The study sampled 2,224 individuals in total. The seroprevalence was estimated at 40.8% (n=998 of 2,447 sampled), and ranged from 58% among adolescents 10 to 14 years old after adjustment (n=25), to 35% among people 50 to 59 years old after adjustment (n=60). The highest infection fatality ratio was estimated at 1.5% (95% CI: 1.4 – 1.6%) among people aged 70 years and above, while adolescents aged 10 to 14 years had the lowest at 0.001% (95% CI: 0.0003 – 0.005%). The overall estimated IFR was 0.24% (95% CI: 0.23 – 0.25%) after age adjustment. This was lower than the CFR, which was estimated at 5.6% (95% CI: 5.3 – 5.9%) overall, and ranged from 26.5% among people 70+ years old (95% CI: 25.0 – 28.0%) to 0.1% among 10 to 14 year old adolescents (95% CI: 0.006 – 0.6%).

Study Strengths

The study used seroprevalence data to estimate the infection fatality rate, rather than only examining among cases of disease for the case-fatality rate. The study also included age-stratified estimates to highlight the changes in these rates. They also used 3 types of immoglobulin as indicative of seropositivity, which will likely capture individuals who may not show an immune response on all measures simultaneously. Notably, they study found a very high seroprevalence overall among those sampled (40.8%).

Limitations

The study was limited by using demographic data reflecting the 2020 population, as opposed to the 2021 population. Similarly, they included deaths from the entire pandemic (March to October 2020), while their seroprevalence data was only derived from August to September 2020. Therefore, these estimates may not be accurate, with an inflated number of deaths due to the longer ascertainment period compared to the seroprevalence period. Additionally, individuals with prior infections may not reflect a detectable antibody response depending on the timing of infection and their immune system, which would reduce the number of seropositive individuals and inflate the estimated IFR. Finally, the model specification for their confidence intervals was not clearly explained, and it is challenging to determine if they used valid statistical methods and sample weighting for their study design, and it is not possible to estimate how biased the results may be from this.

Value added

The study reports a high seroprevalence of SARS-CoV-2, including among children, in South America.

This review was posted on: 17 July 2021