Sweden overestimated asymptomatic Covid-19 cases, says country’s top infectious diseases specialist

Dr Anders Tegnell

Credit: Pontus Lundahl/EPA-EFE/Shutterstock

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Sweden overestimated the number of asymptomatic Covid-19 cases, according to the country’s top infectious diseases specialist.

Dr Anders Tegnell, Sweden’s state epidemiologist who devised the no-lockdown approach, said the level of immunity populations had against the virus was "an enigma and mystery" in the early stages of the pandemic.

Covid-19 was perhaps falsely compared with flu, when a large proportion of people catch it and show no symptoms, recover, then develop antibodies, said. Dr Tegnell.

This resulted in Sweden believing more people had developed immunity than was accurate, he explained.

"We thought that it would be similar with Covid-19 — that we would have quite a large part of people that actually develop antibodies with very low levels of symptoms and didn’t show up in health care," Dr Tegnell told the Oxford Union this week.

"That has proven to be not right because immunity in the population has developed much much lower than I think anybody expected in the beginning. A lot of the initial modelling stuff was also having that assumption and it didn’t turn out to be the case.

"I think what happened is that we overestimated the number of people that would have this disease without symptoms, and that led us to believe that immunity in a population would develop much quicker than it did."

Immunity against Covid-19 is a subject which has divided scientists, with disputes over how it is acquired, how long it can last, and whether asymptomatic carriers can transmit the virus to others.

However, Dr Tegnell emphasised that Sweden was not rigorously pursuing herd immunity — when enough people in a population have been infected and recovered, reducing the chance it will be transmitted to uninfected individuals.

"Herd immunity is not even possible to attain without having a vaccine, because you can never reach those high levels of immunity that actually stops the disease," he said.

"But on the other hand, we thought it was needed to discuss immunity in the population also because this is one of the factors that will determine how the spread will develop over time.

"With this disease, it has been a lot more difficult to understand the connection between the level of immunity in the population and the extra spread of disease because there are communities now that have a very high level of immunity and they still have outbreaks of the disease. Obviously, there is something that we do not really completely understand."

Sweden adopted a very different strategy to tackle Covid-19 compared with many other countries, choosing to avoid a full-blown lockdown.

In addition to keeping schools open, shops and restaurants also remained open across the country, with parks and recreational areas packed with groups enjoying the spring sunshine.

The government only banned events with more than 500 participants, issuing a recommendation that those who visit pubs and restaurants should be seated at a table rather than mingling at a bar.

People taking public transport were advised to "think carefully" about whether their journey is necessary. Those who fell ill with coronavirus-like symptoms only needed to wait two days after they felt well again before returning to work or school.

Dr Tegnell said Sweden potentially underestimated the second wave of the virus in December.

"After the summer it looked fairly promising and we had a low level of cases, for longer than most other countries, but when it hit Sweden it hit us very badly," he said.

In the country’s population of 10 million, there have so far been more than 600,000 Covid-19 cases and 12,428 deaths.

Last month, Sweden tightened social distancing rules for shopping centres, gyms and private gatherings and said it was ready to close businesses if needed, but stopped short of a lockdown.