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A good 80 percent of people who have already been infected with the coronavirus are protected from being infected again for at least five or six months.
This is what researchers report in the journal “The Lancet” based on data from Denmark.
However, this natural protection is less pronounced in the elderly: According to the study, it was only around 47 percent.
It is quite possible that the protection will last longer than six months.
"We did not find any indications in the study that the protection against renewed infection decreases within the six months," says the researcher Daniela Michlmayr from the Statens Serum Institute in Copenhagen, who was involved in the work.
"With the closely related coronaviruses Sars and Mers, the immune protection lasts for up to three years."
Almost 70 percent of Danes had themselves tested
The research group analyzed data collected as part of the Danish Covid-19 testing strategy.
The country offers PCR testing for everyone, regardless of symptoms.
In 2020 around four million people, almost 70 percent of the people in Denmark, had themselves tested.
During this period, Denmark experienced a first and second wave of infections.
Among those who had no proven infection in the first wave between March and May, 3.3 percent received a corona diagnosis in the second wave between September and December.
Of those who were verifiably infected with corona in the first wave between March and May, on the other hand, only 0.65 percent received a corona diagnosis again between September and December.
In the subgroup of people aged 65 and over, the distribution looked different: while in the second wave exactly two percent of senior citizens received a corona diagnosis for the first time, 0.88 percent of those infected in the spring now received a positive test result again.
A second analysis of infections that occurred outside of the second wave produced very similar results.
Steen Ethelberg, also from the Statens Serum Institute, sees this as confirmation that older people have a higher risk of being infected again after a coronavirus infection.
"Because older people also develop severe symptoms more often or even die, the result underscores the importance of protecting the elderly during the pandemic."
The data evaluated by the research group did not contain any information about the severity of the disease.
So it cannot be deduced from this whether, for example, a second infection may be milder or whether an asymptomatic corona infection protects against re-infection just as well as a Covid 19 disease.
In individual cases, those affected who received a positive test result in both corona waves could also have had a certain viral load in their body throughout; they would then be incorrectly classified as re-infected.
Overall, from the point of view of the research group, the result speaks in favor of vaccinating people even after they have had a corona infection, because natural vaccination protection alone cannot be relied on.
No statement about new virus variants
The Standing Vaccination Commission in Germany advises that people who have gone through an infection should not be vaccinated initially - due to their immunity and the existing lack of vaccines.
Therefore, “a COVID-19 vaccination should be considered six months after recovery or diagnosis at the earliest, taking the prioritization into account”.
Then a single dose of vaccine is enough.
It is not yet possible to say whether and when a second corona vaccination will be necessary later.
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The Danish study does not allow any conclusion to be drawn about the natural protection that a previous infection offers against new virus variants.
Because some of them seem to manage to evade the defenses built up by the immune system better.
This applies to variant B.1.351, which was first detected in South Africa, and to variant P.1, which became known in Brazil.
Both appear to reduce the effects of neutralizing antibodies that are produced after infection or vaccination.
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