In an email sent on March 14th, three days before Sweden closed down upper secondary schools and universities
, Sweden's state epidemiologist Anders Tegnell wrote to Mika Salminen, his Finnish counterpart, forwarding an email from a retired doctor who suggested allowing healthy people to get infected voluntarily and in controlled settings.
Salminen replied that his agency had rejected the idea after considering that allowing a spread between children would also increase the rate of spread elsewhere in society.
“We have also considered that, but over time the children are still going to spread the infection,” wrote Salminen.
“True,” Tegnell replies, “but probably mostly to each other because of the extremely age-stratified contact structure we have”.
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The emails are interesting because Tegnell has consistently argued in public that as well as generally receiving only extremely mild symptoms, children do not spread coronavirus to any great degree, even to each other, which would mean holding schools open would have little impact on levels of immunity among the general population.
“My comments were about a possible effect, not an expected one, which was a part of the assessment of the suitability of the measure,” he said. “Keeping schools open to achieve herd immunity was therefore never on the table.”
The emails reveal how Swedish officials discussed herd immunity – which they have argued was not the objective of the strategy, albeit a possible consequence – among themselves and their colleagues. But they are also hard to interpret, written in brief sentences and not always with clear punctuation.
Another email from Tegnell suggests that he did not believe children are the main spreaders of the coronavirus, which unlike the above email seems to support the claim that keeping schools open was not done to achieve herd immunity.
The day after his email to Salminen, Tegnell replied to his predecessor as state epidemiologist, Annika Linde, who had sent him an email arguing that children, who can be heavily affected by flu, appeared to be “symptom-free spreaders” of coronavirus.
“This can ease the build-up of at least temporary immunity in the population…,” she wrote to him. “But nearly asymptomatic spreading also increases the risk of spreading to risk groups.”
In his reply, Tegnell questioned her argument.
“The role of children in this epidemic is hard to understand, I think. Evidently they are not the motor of the epidemic as with influenza.”
Herd immunity occurs when the spread of a virus slows down to the extent that so many people become immune to a disease that the whole population in effect becomes 'immune', even if they have not had the virus. It can be achieved either through vaccinations or infections, or a combination of both.
Much remains unclear about immunity and the coronavirus, since it is such a new virus. The World Health Organisation said in April that antibodies would likely “provide some level of protection” against re-infection. But there is still no conclusive evidence to show whether people are fully immune, or for how long.
The Swedish Public Health Agency has said people who have been diagnosed and recovered from the coronavirus are likely immune from re-infection for at least six months, but advises people to keep following recommendations, including staying home when showing any symptoms and keeping social distance.