Covid-19: Researchers find higher than expected reinfections with P.1 variant among the Brazilian Amazon

A study of blood donors in the city of Manaus has found that 1 in 6 of those infected with the coronavirus were reinfected with the P.1 variant.1 The results were higher than researchers anticipated and those found in studies of reinfections conducted elsewhere.

The preprint, which has not been peer reviewed, was published by the Brazil-UK Center for Arbovirus Discovery, Diagnosis, Genomics, and Epidemiology (CADDE). It is the latest effort from researchers to understand why hospitals in Manaus—which is home to two million in the Brazilian Amazon—collapsed under the weight of coronavirus infections in January 2021 despite antibody levels suggesting that around three quarters of the population had been infected in the first wave.2

The study suggests that reinfections with P.1 are more common than researchers had previously thought and confirms the theory that herd immunity through natural infection is impossible, said Ester Sabino, the study’s lead author and associate professor at the Institute of Tropical Medicine at the University of São Paulo School of Medicine.

Sabino’s team examined antibody levels in 240 blood donors who were not vaccinated against covid-19 and gave blood regularly. Of those who had been infected, 16.9% had a subsequent spike in antibody levels, suggesting they were infected again.

The actual number of reinfections were likely higher, but some of the blood samples were not frequent enough to confirm suspected cases, the authors say. For “probable reinfections” the rate was 25.8%, and for “possible reinfections” it was 31.0%.

The findings suggest that “reinfection due to P.1 is common and more frequent than has been detected by traditional epidemiologic, molecular, and genomic surveillance of clinical cases,” say the authors.

Few studies of reinfection rates have been conducted by looking at antibody levels. Most have sought to prove reinfections with two positive polymerase chain reaction (PCR) tests. But such cases are rare and hard for scientists to confirm, particularly in countries such as Brazil where there is insufficient access to testing. “The chance that you have a person that at both times is symptomatic and PCR positive restricts the chance of finding cases,” Sabino told The BMJ.

Manaus has been labelled a sentinel population as it was believed to have been the first city in the world to reach herd immunity before it was devastated by an unexpected second wave of infections in early 2021. A leading theory has been that P.1—which emerged there in November 2020—drove a second wave by causing reinfections and spreading more easily. P.1 subsequently spread throughout Brazil and drove the country’s deadliest wave of infections yet.

CADDE’s study supports the reinfection theory and confirms that P.1 is better at evading antibodies in those infected with an older variant, as has been suggested in laboratory studies.3 “P.1 has all those mutations that are associated with an escape mutant, plus it has a mutation that increases transmissibility. Both things together, along with the local population’s behaviour, were responsible for the increase in the number of cases,” Sabino said. “It’s concerning because we might have the same thing happen again, people will start losing their immunity and may be reinfected.”

The team is now contacting donors to establish how many cases of reinfection were symptomatic and their degree of severity.

The number of positive coronavirus cases has risen consistently for four weeks in Manaus, fuelling fears of an imminent third wave of infections, according to Jesem Orellana, a local epidemiologist at the Oswaldo Cruz Foundation (Fiocruz).

Infections are currently around 20 for every 100 000 people, similar to levels at the end of 2020, a month before the city’s hospitals collapsed and its oxygen supplies were exhausted.

Vaccines are being administered across Brazil but a lack of supplies is slowing the rollout. Production of Coronavac vaccines was reportedly halted in São Paulo last week as materials did not arrive from China. Some Brazilian senators have alleged that Brazil’s treatment of China hampered its access to global vaccine supplies and São Paulo’s governor has blamed the latest delay on President Jair Bolsonaro for causing a diplomatic spat with China.

The allegations come amidst an official investigation into the Brazilian government’s failure to manage the pandemic, which has killed at least 450 000 people.