The researchers estimated a 76 per cent attack rate in Manus by October, including adjustments for waning antibody immunity.
Brazil has experienced one of the world's most rapidly growing COVID-19 epidemics, with the Amazon being the worst hit region (AP Photo)
Scientists have analysed the spread of COVID-19 among people in Manaus, Brazil, where more than 70 per cent of the population was infected within seven months of the novel coronavirus arriving in the city, findings which shed light on what may happen if the disease spreads unmitigated.
Brazil has experienced one of the world’s most rapidly growing COVID-19 epidemics, with the Amazon being the worst hit region, according to the researchers, including those from Harvard University in the US. They said in Manaus, the largest metropolis in the Amazon, the first SARS-CoV-2 case was reported in mid-March, after which non-pharmaceutical interventions (NPIs), such as social distancing, 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, the study, published in the journal Science, noted. 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, the scientists collected data from blood donors in Manaus.
They inferred the virus attack rate from the collected blood samples, and compared this data with that of Sao Paulo, which was less impacted. The researchers estimated a 76 per cent attack rate in Manus by October, including adjustments for waning antibody immunity. By comparison, the attack rate in Sao Paulo by October was 29 per cent, partly explained by the larger population size, they added.
Despite the tremendous toll the virus took in these two cities, the scientists said 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 noted, also acknowledging voluntary behavioral changes as helping. However, the scientists believe further studies in the region are “urgently” needed to determine the longevity of population immunity.
“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,” they wrote in the study