The immunity of Covid herds remains a mystery to science

The man behind Sweden’s strategy to fight the coronavirus, best known for the lack of quarantines, says key questions remain about how immunity works. Anders Tegnell, Swedish State Epidemiologist, says it is not yet clear to what extent transmission rates decline when more people have been exposed to the virus.

It is “very difficult” to understand, he said in an interview on Tuesday, after speaking with the press.

Tegnell’s doubts about immunity come after the pandemic developed after the summer, while Sweden has seen increased transmission rates since then, as have other parts of the northern hemisphere. Herd immunity occurs when a sufficient number of people in a community have been infected or vaccinated and are therefore immune. In the case of covid, it is estimated that it is achieved when the percentage of immunity is between 55% and 82%.

Data released by the Swedish National Health Agency in June indicated that around 10% of the population of Stockholm, the worst-affected area in Sweden, had developed antibodies to COVID-19. Since last week, more than 70,000 confirmed cases and nearly 100,000 antibody tests positive in the Stockholm region, which has a population of 2.4 million.

The balance

“Obviously this slows down transmission, but it has been difficult to understand the magnitude of this effect and how it should be balanced against other factors that speed up transmission,” Tegnell said. This “balance may have been different from what I and many others believed.”

Sweden has made international headlines to avoid quarantines since the start of the pandemic, relying mainly on voluntary measures to achieve social distancing. It’s a strategy that coincides with a significantly higher death rate than in other parts of the Nordic region, and the government recently recognized that tighter restrictions were needed.

Although Sweden has never targeted herd immunity as a stated strategy, officials have made it clear that they expect some form of resistance to develop in the population that will limit the spread of COVID -19.

Part of the difficulty in predicting immunity levels lies in the initial flawed assumptions that the number of people who had been infected with the virus was higher than what official data showed, according to Tegnell.

“What we have tried to achieve from the start is to keep transmission as low as possible with measures that have the least possible adverse effect on public health,” Tegnell said. “And that’s what we’re still doing ”.