What does Sweden's lower-than-expected immunity mean for the future of its strategy?

At a press conference last week, state epidemiologist Anders Tegnell acknowledged that immunity levels are likely lower than previously hoped for, saying: "The development has been different from what we believed in the summer, and that’s not just the case for Sweden but for the whole world."

He also said that the number of cases not detected was "with high probability smaller than
we thought," meaning that any immunity was not enough to counteract the changes in both weather and behaviour as people returned to work and indoor socialising.

It is still unclear what percentage of the population needs to be immune in order to reach herd immunity, but estimations for Covid-19 lie somewhere between 40 and 70 percent -- though immunity may have a beneficial effect before reaching that level.

Herd immunity has never been an official objective of the health agency's strategy. It has however often been mentioned as a probable and beneficial side-effect of Sweden's generally looser measures, giving the country an advantage against a virus that scientists agree we will have to live with in some way for a long time.

In spring, the Public Health Agency predicted that Sweden would be spared a steep incline in cases in autumn, as Sweden would have reached a considerably higher degree of immunity than, for example, its neighbouring countries Norway and Finland.

In late summer, the world's attention was once again focused on Sweden as coronavirus cases dropped to very low levels. The spread of the virus had gone down spectacularly despite an uptick in testing, and so had the daily number of deaths.

But after the decline over the summer, the spread of infections has risen sharply again since the beginning of October. The number of daily casualties is also on the rise now, having reached the double digits in early November. Compared to Norway and Finland, both cases and deaths have been higher per capita in Sweden.

Annika Linde, who preceded Tegnell as state epidemiologist, told The Local she had hoped immunity would limit the severity of a second wave, but also said more measures should have been taken before the spread reached its current level.

"I still think it's very, very important to act very strictly in the beginning [of an upward curve of infection] and that's where so many countries, including Sweden have gone astray. When the climate, as it is now, becomes suitable for spreading -- climate combined with changed behaviour -- if you then have enough persons infected who can spread the virus, then they start becoming spreaders when the environment becomes suitable," she explained.

Fredrik Elgh, a professor in virology at Umeå University, told The Local Sweden: "Had the virus behaved like the flu and had we been able to protect all vulnerable people, then the virus would have swept through the country in around three months. After which we would have been immune."

But it was quickly apparent that the virus behaved differently, Elgh said.

The spread was uneven: in some regions it barely spread at all, while some ‘super spreaders’ caused (local) spikes in infections elsewhere. Moreover, many of the people who weren’t considered among the vulnerable groups still became much sicker than expected.

So does this mean Sweden should tighten its strategy heading into winter to reflect with what we now know about the disease?

Elgh, one of a group of scientists that has long campaigned for stricter measures, believes recent new recommendations don't go far enough because of the low level of immunity in Sweden.

He argues that the most recently presented measures, including local recommendations and a ban on public events over eight people, are now not enough to really stem the spread.

"At this stage we should use all the tools in our toolbox. We should be wearing face masks, kids in middle school and high school should be taught online for three or four weeks, as should all higher education students. We should increase testing and tracing, while quarantining much more people than we currently do. Children of parents who have tested positive are now going back to school. That is absolutely crazy," he said.