Herd immunity is a mystery, Sweden’s top epidemiologist says

Epidemiologist Anders Tegnell says herd immunity does slow down virus transmission but it hasn't been clear to what extent the rates are reduced and how large that effect it.

A healthcare services sticker in Stockholm instructs people to follow the two-metre rule during the Covid crisis | Representational image: Jonathan Nackstrand | Getty Images via Bloomberg

Stockholm: The man behind Sweden’s coronavirus strategy, best known for the absence of a lockdown, says key questions remain about how immunity works.

Anders Tegnell, Sweden’s state epidemiologist, says it’s still not clear to what extent transmission rates are reduced when more people have been exposed to the virus.

It is “very hard” to understand, he said in an interview on Tuesday, after briefing reporters.

Tegnell’s doubts about immunity follow developments after the summer, with transmission rates since then surging in Sweden and elsewhere in the northern hemisphere. Herd immunity comes when enough people in a community have either been infected or vaccinated, and are thus immune. For Covid, the percentage is estimated to range from 55% to 82%.

Data published by Sweden’s national health agency in June indicated that about 10% of people in Stockholm — Sweden’s worst affected area — had developed antibodies to Covid-19. As of last week, there’d been more than 70,000 confirmed cases and close to 100,000 positive antibody tests in the Stockholm region, which has a population of 2.4 million.

The balance

“It’s obvious that it does slow down transmission, but it’s been difficult to understand how large that effect is and how it should be weighed against other factors that speed up transmission,” Tegnell said. That “balance may have been different than I and many others believed.”

Sweden has made international headlines for avoiding a lockdown since the pandemic hit, relying mostly on voluntary measures to achieve social distancing. It’s a strategy that’s coincided with a death rate that’s multiples of that recorded elsewhere in the Nordic region, and the government has recently acknowledged stricter curbs are needed.

Though Sweden never targeted herd immunity as a declared strategy, officials made clear they expected some form of resistance to build in the population that would limit the spread of Covid-19.

Part of the difficulty in predicting immunity levels lies in false assumptions early on that many more people were infected than official data showed, according to Tegnell.

“What we have tried to achieve from the beginning is to bring transmission as low as possible with measures that had as little adverse effects as possible to public health,” Tegnell said. “And that is what we are still doing.”- Bloomberg