Denmark Study: COVID-19 Patients Are About 80% Protected from Reinfection

March 17, 2021
Kevin Kunzmann

The new findings support increased protection from reinfection up to 6 months after first testing positive, though older patients are at greater risk of reinfection.

A majority of patients with COVID-19 are protected from reinfection over a span of at least 6 months, according to a new real-world assessment from Denmark.

In new research published in The Lancet Wednesday evening, investigators from the Statens Serum Institut in Denmark reported that just 0.65% of approximately 4 million PCR test-positive COVID-19 patients returned another positive test in 2020. The findings would indicate that persons under the age of 65 years old have an approximate 80% likelihood of protection against COVID-19 reinfection—while among older patients, protection was just 47%.

What’s more, the evidence that natural protection against COVID-19 reinfection did not decline over a half-year bodes well for an increasingly vaccinated global population without long-term prophylaxis data indicating immunity duration.

On the matters of increased reinfection risk among persons aged 65 years and older—a patient population already at heightened risk of COVID-19 severity and mortality—study author Dr. Steen Ethelberg said the findings support the pandemic-long message of protecting the elderly.

“Given what is at stake, the results emphasize how important it is that people adhere to measures implemented to keep themselves and others safe, even if they have already had COVID-19,” Ethelberg said in a statement. “Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions.”

Ethelberg and colleagues conducted the analysis of a national COVID-19 testing database that included 69% of Denmark’s population, from PCR test results registered in 2020. The research prominently spanned first and second waves of new COVID-19 cases in the country.

Investigators estimated protection against repeat infection from the previously dominant SARS-CoV-2 strain at the time of the first waves—meaning that their research did not consider risk of reinfection due to more transmissible variants. Differences in age, sex, and time since infection were accounted for assessment of positive and negative test result ratios, which then helped produce estimates of protection against COVID-19 reinfection.

Among persons infected with COVID-19 between March – May 2020, just 72 (0.65%) of 11,068 tested positive again during the second wave from September – December 2020. The reinfection rate was just 0.60% for those under 65, and 0.88% for those 65 years and older.

An alternative cohort analysis of approximately 2.5 million person’s tests results also showed an estimated reinfection protection of 78.8%. The rate was 80.5% for persons under the age of 65, and 47% for those 65 and older.

A sub-analysis of healthcare worker reinfection risk, due to their high exposure rate, showed just 8 (1.2%) of 658 workers who tested positive were reinfected. Their estimated reinfection protection was 81.1%.

Due to their high risk of exposure to the virus, a sub-analysis of healthcare workers was also carried out. Again, results were similar to those of the main analysis, with 1.2% (8/658) of those who had COVID-19 during the first wave becoming re-infected, compared with 6.2% (934/14,946) of those who were negative during the first wave. Estimated protection against reinfection was 81.1%.

Though the findings are promising for the prospect of natural protection following recovery, investigators stressed they do nothing to lessen the importance of social distancing and vaccination. If anything, it stresses those practices for older, at-risk populations.

Observation of sustained protection against reinfection will continue among previously infected patients, study author Dr. Daniela Michlmayr, said in a statement.

“In our study, we did not identify anything to indicate that protection against reinfection declines within six months of having COVID-19,” Michlmayr said. “The closely related coronaviruses SARS and MERS have both been shown to confer immune protection against reinfection lasting up to three years, but ongoing analysis of COVID-19 is needed to understand its long-term effects on patients’ chances of becoming infected again.”