Herd Immunity Effects on COVID-19 in Hard-Hit Brazilian City

Herd Immunity Effects on COVID-19 in Hard-Hit Brazilian City

Author: ChemistryViews.org

The coronavirus SARS-CoV-2 causes the COVID-19 pandemic. Control measures have been implemented to limit the damaging effects of the pandemic. However, only significant immunity in the population will lead to a sustained decline of infections and the end of the pandemic without such control measures.

This so-called herd immunity could be reached by efficient and widespread vaccination campaigns or by the natural spread of the infection. The latter depends on how long the immunity to COVID-19 lasts after an infection. Achieving herd immunity by unchecked, natural spread of COVID-19 would result in significant loss of life. The exact proportion of a population that must be immune to stop the spread of the infection is not known, but has been estimated to be about 60 % using simplified models.

Nuno R. Faria, University of São Paulo, Brazil, University of Oxford, UK, and Imperial College London, UK, Ester C. Sabino, University of São Paulo, and colleagues have used samples from blood donations to estimate the population immunity to COVID-19 in Manaus, Brazil. The city is the capital of the worst-hit region in Brazil and has experienced high numbers of COVID-19 infections. The first case in the city was confirmed in March 2020. The infection spread rapidly and the epidemic peak was reached in early May. Then, the number of cases dropped despite less strict control measures, indicating possible herd immunity. The team used blood samples from the months of February to August 2020 and tested them for antibodies against the SARS-CoV-2 virus using a commercially available chemiluminescence assay.

The team found that one month after the peak of the epidemic, 44 % of the samples tested positive for SARS-CoV-2 antibodies. Corrected for the false-negative rate of the antibody test, this means 52 % of the population would have been infected. However, antibody levels decrease over time after a COVID-19 infection, leading to negative test results for samples from people who had the infection. This means the actual number of COVID-19 cases in the region should be even higher. The team estimates a final epidemic size of 66 % of the population.

This estimate’s accuracy is limited by the quality of the antibody tests, the assumptions made to compensate for the drop in antibody levels, and by how well blood donors represent the overall population. The team cautions that it cannot be easily transferred to other regions due to differences in behavior and vulnerability to the infection. Nevertheless, further monitoring data from Manaus could, for example, be useful to determine how long population immunity lasts.

  • COVID-19 herd immunity in the Brazilian Amazon,
    Lewis F. Buss, Carlos Augusto Prete, Claudia M. M. Abrahim, Alfredo Mendrone, Tassila Salomon, Cesar de Almeida-Neto, Rafael F. O. França, Maria C. Belotti, Maria P. S. S. Carvalho, Allyson G. Costa, Myuki A. E. Crispim, Suzete C. Ferreira, Nelson A. Fraiji, Susie Gurzenda, Charles Whittaker, Leonardo T. Kamaura, Pedro L. Takecian, Márcio K. Moikawa, Anna S. Nishiya, Vanderson Rocha, Nanci A. Salles, Andreza A. de Souza Santos, Martirene A. da Silva, Brian Custer, Manoel Barral-Netto, Moritz Kraemer, Rafael H. M. Pererira, Oliver G. Pybus, Michael P. Busch, Márcia C. Castro, Christopher Dye, Vitor H. Nascimento, Nuno R. Faria, Ester C. Sabino,
    medRxiv 2020.

The research has been published as a preprint and has not yet been peer-reviewed. It should not be used to guide clinical practice.

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