Herd immunity to Covid19 – new study suggests Brazilian city has it

In May bodies were piling up so fast in Manaus, Brazil, that authorities struggled to bury them all. Coronavirus has torn through Brazil, leaving it with the second worst death rate in the world, and nowhere was worse hit than the Amazonas state where Manaus is located. At one point the death rate hit 19.4 per 100,000, nearly five time worse than the national average.

However, since June the number of new cases and deaths in Manaus has dropped sharply, from a peak of 79 deaths a day to two or three, even as anti-pandemic measures relaxed. This has prompted some to speculate that the city had developed herd immunity.

Now a new study by an international team of researchers, released on medrvix but still pending peer review, has suggested this might well be the case. Using a mix of antibody testing and mathematical modelling they estimated 44-66% of the city’s population was infected by coronavirus. Now the researchers believe the virus has simply started to run out of people to infect.

In the report they say: “Although nonpharmaceutical interventions, plus a change in population behavior, may have helped to limit SARS-CoV-2 transmission in Manaus, the unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic.”

Still, while the virus may have burnt itself out, its toll while it lasted was steep. Some estimate the city’s death rate from coronavirus was somewhere between 1 in 500 to 1 in 800. The city is also overwhelmingly youthful, only 6% of the population is aged over 60. In cities with larger populations of elderly people, who are far more vulnerable to the disease, the toll would likely be markedly steeper.

The study provides no information on how long immunity to Covid-19 lasts. If Manaus does indeed have herd immunity there is, for the moment, no telling how long it will last raising the spectre the disease might yet return to ravage the city again.