As of today, nearly 10% of the entire US population has tested positive for SARS-CoV-2. It is very likely that the true number of people infected during this pandemic is substantially higher. An important question emerges: if infected, are we protected? There has been considerable discussion over the past 4 months on COVID-19 vaccine effectiveness, but what is the effectiveness and durability of prior infection?
To answer this question, researchers in Denmark made use of a nationwide registry of PCR results from SARS-CoV-2 testing. Starting with individuals who were tested during Denmark’s first COVID-19 wave (before June 1, 2020), the researchers then evaluated PCR results from the same individuals during the second wave (September 1 through December 31, 2020). Accordingly, they identified 11,068 positive cases and 514,271 negative cases in the first wave. During the second wave, those who were previously positive had an infection rate of 5.4 cases per 100,000 person-days compared with 27.1/100,000 for the PCR-negative group. This resulted in an estimated protection of 80.5% (95% CI, 75.4–84.5). A similar result was obtained when only those individuals who had repeated testing based on occupation were considered. There were no differences based on sex or amount of time in follow-up. Of note, however, was a drop-off in protection for those over age 65 years (down to 47.1% protection).
Observational studies are always a bit risky; but this one was done well and utilized a number of alternative approaches to help validate the results. The period of time between the first and second wave assessment periods was 92 days, so the results apply to a moderate time scale.
The take-home messages for primary care clinicians are as follows:
- Infection with SARS-CoV-2 results in future protection that lasts at least 3 months and appears not to decline over 6 months.
- The protection is comparable to that attained through vaccination.
- There is reduction in protection for our older patients; prioritization of vaccine to individuals aged ≥65 years is reasonable, even when there is a past history of COVID-19.
- Additional research is needed to understand the longer-term durability of protective immunity following infection.
We will need the combined forces of immunity acquired by past infection and through active vaccination to achieve the needed level of herd immunity. The potential protection offered by both routes, however, is promising.