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Seroprevalence of anti-SARS-CoV-2 antibodies in Iquitos, Peru in July and August, 2020: a population-based study

Our take —

This study describes the seroprevalence of SARS-CoV-2 infection in Iquitos, Peru in July and August 2020. In a baseline July rapid testing round, researchers estimated that the population seroprevalence was 70%. During the August rapid testing round, they estimated seroprevalence to be 66%. While this study may have had reduced sensitivity to detect cases due to the use of rapid testing, the results suggest that Iquitos, Peru has one of the highest seroprevalences of SARS-CoV-2 in the world. Depending on the duration of natural immunity, the seroprevalence reported here would surpass estimated herd immunity thresholds for SARS-CoV-2.

Study design

Prospective Cohort

Study population and setting

This study sought to describe the seroprevalence of SARS-CoV-2 infection and estimate transmission among a population-based cohort in Iquitos, an area of Peru with one of the highest mortality rates from COVID-19. To be eligible, respondents had to have lived in Iquitos at least since SARS-CoV-2 was first identified in Peru on March 16, 2020. The study excluded people in institutions, people receiving treatment for COVID-19, people unable to give blood, and health workers or individuals living with a health worker. Participants were enrolled from July 13 to July 18, 2020, with one follow-up test between August 13 and August 18, 2020. Stratified population sampling was conducted across four levels (four districts, 40 sectors, 2,500 blocks, 90,354 households). Participants aged 13 years and older were eligible, provided guardians consented for participants aged 13 to 17 years. In total, 726 individuals were eligible and 716 participants were enrolled (99%). After one month, 621 participants were tested again (87% of 716). Seroprevalence was estimated using IgG and IgM antibodies assessed via rapid test. Surveys were also conducted to collect data on sociodemographic characteristics.

Summary of Main Findings

At baseline in July 2020, 528 of 716 (74%) participants tested positive for IgM (7%, n=50) or IgG (74%, n=526) SARS-CoV-2 antibodies. After adjusting for sampling procedures, the estimated population seroprevalence was 70% (95% CI: 67 – 73). After follow-up, 621 participants were tested, with 422 (68%) seropositive for either IgM or IgG. The adjusted seroprevalence was 66% (95% CI: 62 – 70%). The incidence of new SARS-CoV-2 seroprevalent cases was estimated at 2% (95% CI: 1 – 3%) in the 1-month follow-up period between test and retest. The test-retest positivity among participants was estimated at 65% (95% CI: 61 – 68%) among those who tested positive at baseline and one month later. Researchers found no difference in seroprevalence between men and women, and among the 619 participants, individuals in urban areas had lower incidence of new cases compared to rural areas (prevalence ratio: 0.21, 95% CI: 0.07 – 0.68).

Study Strengths

This study had data available among almost all eligible participants (99%) and a high retention rate (87%), which reduced the likelihood of selection bias. Additionally, the researchers used survey weights to estimate the population-level seroprevalence, which adds further robustness to the estimates. The study also tested for multiple immunoglobulin types (i.e., IgG and IgM) which captured individuals who would have been missed if they had used a single testing type.

Limitations

This study used rapid tests, which may be less sensitive than a polymerase chain reaction (PCR) test. Therefore, some people with previous infections may have been missed and these results would be an underestimate of the true seroprevalence of SARS-CoV-2. These findings are also impacted by waning immunity, in which individuals who initially have evidence of IgG and IgM antibodies may have reduced or even undetectable antibody levels over time. If participants had previous SARS-CoV-2 infections but undetectable antibodies, then the results presented here would also be an underestimation of true prior infections.

Value added

This study, from a major city in Peru, reports one of the highest seroprevalences of SARS-CoV-2 in the world.

This review was posted on: 27 June 2021