The good people of Denmark have once again provided their excellent, centralized healthcare data to save us all, this time allowing researchers to track the COVID-19 infection rates of 4 million Danes last year, to see how many were infected twice. The results were published in The Lancet last week.
The study found that just 0.65% of people who tested positive for COVID-19 in the spring were reinfected later in the year. In context, this number is both surprisingly high and surprisingly low: It means that most people will not get reinfected—but when you factor in the country’s low 2% infection rate, it means that *of the Danes who are actually twice-exposed to COVID-19*, a surprising number of them become reinfected. Among young and middle aged Danes, their re-infection rate could be around 1 in 5—which is significant, if not alarming.
One group did display a rather low protective rate: people over age 65, who had only a 47% protection rate. “Natural protection, especially among older people, cannot be relied on,” wrote the researchers, who call for continued social distancing around people over age 65, as well as vaccinations.
Notably, protection against repeat infection continued for at least six months.
Some caveats: This is the first large-scale study on reinfections, a topic that is not well-understood by scientists. This paper essentially sketches out that reinfection is happening on a notable scale in one country, but does not delve into the details. For example:
- The study did not trace strains of COVID-19. (It’s possible that all the reinfections were from a particular variant of the virus. Or not. We have no idea.)
- The researchers also could not track symptoms. (It’s possible that many of the people who tested positive twice were not symptomatic both times. We don’t know.)
Mostly, the paper demonstrates that more research is urgently needed. This work was carried out by two universities in Denmark, as well as the European Centre for Disease Prevention and Control.