Researchers tested 97,950 blood donor samples for immunoglobulin G (IgG) antibodies. Samples were collected in eight of Brazil’s most populous state capitals: Belo Horizonte, Curitiba, Fortaleza, Manaus, Recife, Rio de Janeiro, Salvador, and São Paulo. The study period lasted from March 2020 to March 2021.
The results showed that the COVID-19 epidemic spread unevenly and affected the population of these cities at different times. Seroprevalence was generally highest among males and young adults.
Previously, some lines of research assumed that everyone was infected at the same time, but we have shown that this is not the case. Regarding the portrait of the epidemic, they concluded that it was extremely heterogeneous in Brazil, with different levels of infection and significant variation in mortality rates between groups.
Blood Donation Patterns Can Be Used to Monitor Progress of Epidemics
In the study, in addition to estimating the disease attack rate, or seroprevalence, over time for eight cities using sex- and age-matched blood donation patterns, the researchers calculated the age-specific infection mortality rate (IFR) and infection hospitalization rate for each city.
The researchers tested 1,000 samples in each city every month. To ensure that the samples were representative, they were selected so that the location of the donors’ homes corresponded to the spatial distribution of the population in the respective areas.
IgG antibodies against SARS-CoV-2 nucleocapsid (N) were detected by chemiluminescence microparticle immunoassay, as this was the only automated kit commercially available in Brazil when the study began (July 2020).
However, this type of test may suffer a loss of sensitivity over time due to declining antibody levels, and the decline in the proportion of individuals with positive test results may lead to an increasingly overestimate of the true attack rate as the epidemic progresses.
Shortly after the study was published, a second wave of COVID-19 occurred in Brazil. At that time, many people thought that there would be no re-infection.
Researchers have now confirmed that blood donation samples can be used to estimate seroprevalence for monitoring other diseases, provided certain adjustments are made, such as correcting for seroreversion and stratifying by age and gender for each group using spatially representative samples. assess the degree.
The results showed that the attack rate varied from 19.3% in Curitiba to 75% in Manaus in December 2020 before the gamma variant became dominant. Seroprevalence was consistently lower among women older than 55 years and among donors.
The infection mortality rate (IFR) also varied between cities, ranging from 0.24% in Manaus to 0.54% in Curitiba, and the IFR increased consistently with age. This approach corrected the effect of underreporting, especially in 2020 when the test was not widely available.
The increasing penetration of COVID-19 and the inability of the Manaus health system to cope with the influx of cases caused the gamma-related IFR to reach a level at least 2.91 times higher than in the first wave.