Thread by @JCedernaes

1) A Danish study @TheLancet has followed over 520 000 people, from the 1st to 2nd surge.

Similar to some other studies, they find that those with a prior #SARSCoV2 infection had ~80% lower risk of #reinfection, but only ~47% reduced in those ≥65 yrs
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thelancet.com/journals/lance…

2) The @TheLancet article describes how Denmark during tested ~0.5 M people for #SARSCoV2 during wave 1.

At the end of 2020, ~10% of the population tested every week.

In all of 2020, Denmark tested ~4 million people (68% of their pop.) and 64% of those had been tested >1 time.

3) In their analysis, they also included an alternative cohort, that looked at individuals throughout the pandemic (& not only those testing positive in the 1st & 2nd wave), thus examining reinfection risk in a group of 2.4 M people, of which 28 875 had been previously infected.

4) In this alternative cohort, they also found a similar reduced reinfection risk (by ~79%; 95%CI 74·9–82·1), and no one infected >2 times.

Similar protection from reinfection also seen in frequently tested #healthcare & social workers (~15.600 individuals) – ~81% risk reduction

5) They were unable to correlate symptoms with the degree of protection. Potential bias could be due to less willingness for previously infected individuals to be tested again (thinking they are protected), resulting in overestimated protection.

6) It is also possible that those with prior infection engage in more risky behavior, which could then lead their risk exposure to be different compared with a previously non-infected individual.

7) The (sensitivity/added) analysis in routinely tested #healthcare workers was to confirm that the reinfection risk was not driven by altered testing behavior in previously infected individuals – as mentioned above, the risk was not much different, suggesting lack of such bias.

8) The authors mention that their study material covers a period when the more transmissible variants of SARS-CoV-2 were not yet established in #Denmark – these may alter reinfection risk per case, but also due to higher community transmission (i.e. higher overall exposure)

9) The risk of reinfection does however not seem to be increased from B.1.1.7:

https://twitter.com/JCedernaes/status/1367394136475721728?s=20

10) The lower protection from reinfection in older individuals seems somewhat different from another much smaller study: in UK care home residents (n=103) & staff (n=106), protection from reinfection was overall ~96%, but again this is a small study.
eurosurveillance.org/content/10.280…

11) There are as mentioned above other studies that have reported a similar level of protection from reinfection, after a previous infection with SARS-CoV-2:

https://twitter.com/JCedernaes/status/1362237441726365698?s=20

12) The authors of the new @TheLancet study highlight that their findings mean older individuals need e.g. to get effective vaccines, and that "vaccination of previously infected individuals should be done because natural protection cannot be relied on"