Swedish Prime Minister Stefan Lofven said he’s in no doubt his country’s highly controversial strategy for fighting Covid-19 remains appropriate.

Lofven, who has seen his popularity flag as Sweden’s decision not to impose a proper lockdown was followed by a spike in deaths, said he still thinks “the strategy is right, I’m completely convinced of that,” according to an interview with Aftonbladet.

The comments follow signs that, despite much higher rates of exposure to the coronavirus in Sweden than in many other places, immunity remains elusive. Meanwhile, Sweden’s mortality rate per 100,000 is higher than that in the U.S.

Immunity Puzzle

Far greater numbers of Swedes have tested positive for Covid-19 than elsewhere in the Nordic region. But Sweden remains a long way off achieving so-called herd immunity, according to the latest data.

“We know that large parts of the population are unprotected, as they haven’t been infected,” Karin Tegmark Wisell, head of the Public Health Agency’s microbiology department, said on July 14. That means there remains a “large susceptibility in the population,” she said.

Data published by the agency in June indicated that about 10% of people in Stockholm—Sweden’s worst affected area—had developed antibodies to Covid-19. In the past four weeks, 17.6% of the more than 140,000 who signed up for free antibody tests in the capital region returned a positive result.

But there are plenty of caveats to figuring out how immunity works when it comes to Covid-19. Data from other coronaviruses suggest that an infected person would have some degree of immunity after recovering, though it’s hard to be sure, according to Maria Van Kerkhove, the World Health Organization’s technical lead officer on Covid-19.

“We do expect people infected would mount some level of immune response,” Kerkhove said at a briefing on July 13. “But we don’t know how strong the protection is or how long it may last.”

In a recent study, researchers at King’s College found that levels of immunity dropped drastically only three months after infection.

Unfortunate Outcome

“Of course, it is unfortunate,” Tegmark Wisell said of the study’s results. “It indicates that the protection from antibodies doesn’t last very long, but there are other parts of the immune system that give protection, for example T-cell response and cellular immunity.”

The concept of herd immunity is a controversial one, and Swedish public health experts have consistently denied that achieving it was ever a stated goal.

Last week, 23 academics who have emerged as the most vocal local critics of Sweden’s Covid-19 strategy, wrote an opinion piece lambasting the country’s approach. They questioned the idea that herd immunity was never a goal, calling it a “stealth strategy,” whereby authorities deliberately sought to expose large numbers of the population to the virus.

Sweden had the highest rate of new cases of all European Union states, barring Luxembourg, in the past couple of weeks. That partly reflects that the country finally, after many false starts, has been able to ramp up testing. However, it also indicates that the the virus remains more widespread than in most countries that have implemented more stringent measures.

The group of 23 critics, many of whom have advocated for a stricter lockdown, say Sweden should now consider a range of measures to save lives. These include quarantines for asymptomatic people who have been in contact with an infected person, and recommendations that face masks be used in public.

Face Masks

The question of face masks has proved particularly thorny in Sweden, with the chief epidemiologist, Anders Tegnell, publicly dismissing their effectiveness early on during the pandemic. More recently, he seemed to backtrack somewhat, but stopped short of recommending their use.

Masks are now recommended by the WHO, and authorities in the U.S. and the U.K. advise that people use them when social distancing is difficult to do.

“Asymptomatic spread and face masks are terms that have been stigmatized, and we need to remove that stigma,” said Bjorn Olsen, a professor of infectious diseases, who is part of the group of 23. He says seeing Anthony Fauci, the director of the U.S. National Institute of Allergy and Infectious Diseases, wearing a face mask sends “a very clear signal, and Sweden shouldn’t believe we have better information than he has.”

Tegnell has argued that the evidence on masks is thin, and says that using them might even create a false sense of security, leading people to neglect essential measures such as hand-washing and social distancing. But a review of 172 studies funded by the WHO concluded that mask-wearing protects against Covid-19 transmission.

Tegmark Wisell said distancing remains the most effective way to prevent infection. But she also said her agency plans to review how to tackle situations in which distancing isn’t an option, “and whether face masks can be of any help.”

Olsen, an early critic of Sweden’s Covid-19 strategy, fears that regardless of immunity levels, it will take a long time for the pandemic to run its course in the country. Though a lockdown would have been the right thing to do when the virus broke out, he says it’s too late now to impose one.

“That train left the station in March, and we dropped the ticket on the platform,” Olsen said. “If we had done it then we would have come out of this very differently.”