Thursday, March 18, 2021

Denmark SSI: Assessment of Protection Against Reinfection with SARS-CoV-2



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Eleven months ago, when the COVID-19 pandemic was still in its early stages, we began looking earnestly at the potential for reinfection with the virus after recovering from the illness (see COVID-19: From Here To Immunity).  

Previous experience with MERS-CoV suggested that those who were mildly - or asymptomatically - infected might not mount a robust and long-lasting immune response (see 2016 study EID Journal: Antibody Response & Disease Severity In HCW MERS Survivors).

While the number of confirmed reinfections has remained small, some of that it due to a lack of testing. Some reinfections, however, have been laboratory confirmed (see MMWR: Suspected SARS-CoV-2 Reinfections Among Residents Of A Skilled Nursing Facility - Kentucky, Jul.- Nov. 2020).
Of growing concern - with the rise of new variants like P.1 and B.1.351 - we are seeing evidence of increased rates of reinfection in hard hit places like South Africa and Brazil (see Brazil MOH Confirms Reinfection With COVID Variant P.1 In Amazonas and The Lancet: Resurgence of COVID-19 in Manaus, Brazil, Despite High Seroprevalence).
While today's study deals with the`wild type' COVID which dominated much of the world in 2020,  results from a large study in Denmark suggest that protection against reinfection with COVID is far from guaranteed, particularly among the elderly. 

Based on 4 million Danes who received multiple PRC tests in 2020, researchers at Denmark's SSI (Statens Serum Institut) have calculated the average person is about 80% protected - at in the short term - against reinfection with COVID-19. 
Among those aged 65 and over, however, that protection is estimated to be only 47%.
This protection is assumed to last at least 6 months.  Once again, these results are based on the older, `wild type' COVID, not the recently emerged variants carrying the E484K mutation, and so these numbers may not hold going forward.  

First we have the (translated) press release describing their findings from the SSI, followed by a link to full study published in The Lancet. 

Can you get covid-19 twice? New calculation shows that 80% are protected against becoming infected again

Researchers at the Statens Serum Institut have made an analysis of the distribution of the more than 10 million PCR tests that were performed in Denmark last year. Calculations show that 80% are protected against becoming infected again. Among the elderly, however, the figure is down to only 50%.
Last edited March 18, 2021

Covid-19 is a new disease and there is still a lot we do not know about it. One of the things that so far has not been sufficiently known about is how much of the population becomes immune after being infected with the covid-19 virus.

Last year, more than 10 million PCR tests were performed in Denmark. Four million of us had at least one test done.

Researchers at the Statens Serum Institut (SSI) have now analyzed how often Danes who were tested with PCR last year were tested positive, respectively, zero, one or more times. The percentage of people who test positive twice at intervals of at least three months can be used to comment on the degree of protection against a new infection after an infection has passed.

The study has been published today in the English medical journal The Lancet.

Only a few were infected again

In the analysis, the researchers compared those who tested positive in the first wave in the spring with the second wave in the fall. The analysis showed that only a few (0.65%) of the first-time positives became positive again in a later test.

If you first had one positive test result, you were approximately five times less likely to be tested positive since then. This corresponds to 80.5% protection against a later infection.

There was no difference in protection between men and women. There was also no difference between the first part and the last part of the time period last year. That is, there was no sign that protection began to wane after six months.
Elderly immunity to covid-19 is far lower

On the other hand, there was a difference when dividing the population into age groups. Among citizens aged 65 and over, the protection is estimated to be 47%.
"We could calculate that the natural immunity is at 80%. But we can also see that the observed immunity is much lower in the elderly, namely only 50% or lower ", says senior researcher Steen Ethelberg. He and colleagues from SSI conducted the study.

Steen Ethelberg continues:
“Our study suggests that most people will be protected from being infected again for at least half a year. But not everyone is protected and especially among the elderly, our study showed something disturbing. That only half were apparently protected. "
"Even if you have already been infected, it would be a good idea to say yes to the offer to be vaccinated. And until then - and this is especially true for the elderly - still remember to follow the authorities' advice on how to protect yourself from infection ", states Steen Ethelberg.

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