It was reported that the Brazilian city, Manaus, likely exceeded the herd immunity threshold (presumably 60–70%) in November 2020 after the first wave of COVID-19, based on the serological data of a routine blood donor. However, a second wave started in November 2020, when an even higher magnitude of deaths hit the city. The arrival of the second wave coincided with the emergence of the Gamma (P.1) variant of SARS-CoV-2, with higher transmissibility, a younger age profile of cases, and a higher hospitalization rate. Prete et al. (2020 MedRxiv 21256644) found that 8 to 33 of 238 (3.4–13.9%) repeated blood donors likely were infected twice in Manaus between March 2020 and March 2021. It is unclear how this finding can be used to explain the second wave. We propose a simple model which allows reinfection to explain the two-wave pattern in Manaus. We find that the two waves with 30% and 40% infection attack rates, respectively, and a reinfection ratio at 3.4–13.9%, can explain the two waves well. We argue that the second wave was likely because the city had not exceeded the herd immunity level after the first wave. The reinfection likely played a weak role in causing the two waves.
【저자키워드】 COVID-19, Reinfection, waning of immunity, boosting of immunity, 【초록키워드】 Immunity, Hospitalization, Infection, Blood donors, Transmissibility, P.1, herd immunity, death, Gamma, second wave, age, First wave, Blood, Donor, Herd immunity threshold, blood donor, attack rates, city, serological data, medRxiv, repeated, variant of SARS-CoV-2, were infected, reported, can be used, magnitude, explain, exceeded, 【제목키워드】 Brazil, SARS-CoV-2, induced,