The results showed that the COVID-19 epidemic spread unevenly, affecting the populations of these cities at different times. Seroprevalence was generally highest among men and younger people.

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Early on, some lines of investigation assumed everyone was infected at the same time, but we showed this wasn’t the case. In terms of a portrait of the epidemic, they concluded that it was extremely heterogeneous in Brazil, with different levels of infection between groups and significant variations in lethality rates.

Blood Donation Samples Can Be Used to Monitor Progress of Epidemics

In the study, as well as estimating the attack rate or seroprevalence of the disease over time for the eight cities using blood donation samples, with a breakdown by gender and age, the researchers also estimated the age-specific infection fatality rate (IFR) and infection hospitalization rate for each city.

The researchers had 1,000 samples tested per city per month. To make sure the samples were representative, they were selected so that the location of the donors’ homes matched the spatial distribution of the population in the areas concerned.

IgG antibodies against the SARS-CoV-2 nucleocapsid (N) were detected by chemiluminescent microparticle immunoassay because this was the only automated kit commercially available in Brazil when the study started (July 2020).

However, this type of test may suffer a loss of sensitivity over time due to waning antibody levels, and the declining proportion of individuals with positive test results may have increasingly led to the true attack rate being underestimated as the epidemic progressed.

Shortly after the study was published, there was a second wave of COVID-19 in Brazil. At the time, many people thought there wouldn’t be reinfection.

Researchers now confirmed that blood donation samples can be used to estimate seroprevalence to monitor other diseases, provided certain adjustments are made, such as correcting for seroreversion and estimating the attack rate for each group by age and gender using spatially representative samples.

The results showed that the attack rate in December 2020, before the gamma variant become predominant, ranged from 19.3% in Curitiba to 75% in Manaus. Seroprevalence was consistently lower among women and donors over 55 years of age.

The infection fatality rate (IFR) also varied among the cities, ranging from 0.24% in Manaus to 0.54% in Curitiba, and the age-specific IFR rose consistently with age. This approach corrected the effect of under-reporting, particularly in 2020 when testing was not widely available.

Increased penetration of COVID-19 and the inability of the Manaus health system to cope with the influx of cases led the IFR associated with gamma to reach a level at least 2.91 times higher than in the first wave.

Source: Eurekalert