Researchers compare data from blood donors to infer the attack rate of SARS-CoV-2 virus

Researchers studying data from blood donors in Manaus, Brazil, who experienced high mortality from SARS-CoV-2, estimate that more than 70% of the population was infected approximately seven months after the virus first arrived in the city.

"Manaus represents a 'sentinel' population, giving us a data-based indication of what may happen if SARS-CoV-2 is allowed to spread largely unmitigated," write Lewis Buss and colleagues. Brazil has experienced one of the world's most rapidly growing COVID-19 epidemics, with the Amazon being the worst-hit region.

In Manaus, the capital and largest metropolis in the Amazon, the first SARS-CoV-2 case was reported in mid-March, after which non-pharmaceutical interventions (NPIs) were introduced. This was followed by an "explosive" epidemic associated with relatively high mortality, and then by a sustained drop in new cases despite relaxation of NPIs.

To explore whether the epidemic was contained because infection reached the herd immunity threshold or because of other factors such as behavioral changes and NPIs, Lewis Buss et al. collected data from blood donors in Manaus - which they used to infer a virus attack rate - and compared it to data from blood donors from São Paulo, which was less impacted.

Analyzing antibody positivity using SARS-CoV-2 IgG tests, the authors estimate a 76% attack rate by October. This estimate includes adjustments for waning antibody immunity. By comparison, the attack rate in São Paulo by October was 29%, partly explained by the larger population size, Buss and colleagues note.

The authors say that, despite the tremendous toll the virus took in these two cities (where transmission is continuing today), the attack rates remain lower than predicted in a mixed population with no mitigation strategies. "It is likely that [NPIs] worked in tandem with growing population immunity to contain the epidemic," they note, also acknowledging voluntary behavioral changes as helping.

Further studies in the region are "urgently" needed to determine the longevity of population immunity, they say.

Monitoring of new cases ... will also be vital to understand the extent to which population immunity might prevent future transmission, and the potential need for booster vaccinations to bolster protective immunity."

Lewis Buss et al, Study Authors