The head of Sweden's lockdown-free coronavirus response, Anders Tegnell, claimed in an interview with The Observer that as many as 30% of the country's population could be immune to COVID-19.
But studies have found much lower rates of antibodies in the Swedish population.
Data in June indicated that about 10% of people in Stockholm, the worst-affected region, had COVID-19 antibodies.
A recent study suggested immunity could exist in more people than have antibodies — but even in light of this many are skeptical of Tegnell's claim
The architect of Sweden's lockdown-free coronavirus strategy claimed that almost a third of the country's population could now be immune to COVID-19 — a theory backed up by little data.
Anders Tegnell, the chief epidemiologist at Sweden's Public Health Agency, told The Observer on Sunday that the recent drop in Sweden's cases could mean there is an immunity level in the population of "20%, 30%, maybe even slightly more in some areas."
Tegnell has been widely credited as the architect of Sweden's unusual response to the coronavirus pandemic, in which the country decided not to institute a widespread lockdown and put in place relatively few restrictions.
He made the claim about immunity while trying to explain the drop in Sweden's cases over the past month.
Tegnell said that seasonal factors like schools and offices closing for Sweden's summer holidays were not on their own enough to explain the drop in cases.
"Exactly why this happened at that time and why it was so quick and sudden, is difficult for us to understand," he said. "But we believe that the increasing number of immune people in the population must have something to do with it."
Despite Tegnell's theory, researchers have so far reached few solid conclusions on immunity to COVID-19.
People who catch a virus usually have antibodies, which can be measured by tests. But it's not clear whether having antibodies offers total — or even partial — immunity to COVID-19, or how long such an effect may last.
Studies measuring antibody levels in Sweden have not been nearly as high as the 20% or 30% cited by Tegnell.
In June, data from its Public Health Agency indicated that about 10% of people in Stockholm, the worst-affected region, had antibodies.
Tentative research has suggested that Tegnell could be right in his claim that immunity is more widespread than the numbers of people with antibodies.
A study from Sweden's Karolinska Insitutet found that immunity levels among populations could be higher than what is indicated by antibody tests.
It noted that another part of the immune system, T-cells, may create immunity even in people with no antibodies.
The study, which is not peer reviewed, suggests that "roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in."
Some experts say there is still too little evidence to know if other functions of the immune system are giving people immunity.
They cite other other likely factors instead, such as people being more distanced than normal during summer holidays.
Virologist Lena Einhorn told Politico that crediting other immune responses in people who don't test positive for antibodies is a "completely unproven assumption."
Sweden's broader expectations for antibodies have not been met.
Tegnell said in April that he expected 40% of people in Stockholm, Sweden's capital, to be immune by the end of May.
A study in late May suggested that 6.1% of Sweden's population had developed coronavirus antibodies.
Tegnell told The Observer the reason that antibody studies had not supported his thesis is that it's hard to get a good sample.
"It's very difficult to draw a good sample from the population, because obviously, the level of immunity differs enormously between different age-groups between different parts of Stockholm and so on, and that's why when we measure one group we get 4% to 5%, and when we measure another group they're up to 25%," he said.
As of Monday, more than 5,700 people had died of the coronavirus in Sweden, a figure far higher than in neighboring countries with similar political systems and social customs.
Its per-capita death figure is more than five times Denmark's, more than 11 times Norway's, and almost 10 times Finland's.
Jan Albert, an infectious-diseases expert at Sweden's Karolinska Institute, told Business Insider last week that Sweden could see further spikes in virus cases, particularly as the summer holidays end.
"We definitely have come down from a peak, but whether we will see an increase again later in the year, especially when workplaces and schools open up again, we don't know," Albert said.
"It's likely that we will see at least smaller outbreaks and possibly some kind of second wave or peak."
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