- According to research, a first infection with SARS-CoV-2 offers just 47% protection against a second infection in people aged 65 and up, compared to 80% protection in people of all ages.
- These findings underscore the importance of physical distancing and vaccinations, even among those who have already had COVID-19.
- The level of defense against reinfection remained stable for more than 6 months, according to the report.
- It did not, however, take into account new virus variants, which could provide less protection against reinfection after infection.
The ease with which people can re-acquire SARS-CoV-2, the virus that causes COVID-19, is a key factor that will decide how quickly life will return to normal after the pandemic.
While confirmed cases of reinfection are exceedingly rare, the most detailed population-based research to date indicates that the risks could be greater than scientists initially assumed.
The study found that an initial infection offered about 80% security across all age groups, based on a registry of millions of polymerase chain reaction (PCR) tests performed in Denmark during 2020.
However, defense against reinfection fell to 47% in people aged 65 and up.
Prof. Steen Ethelberg, Ph.D., of the Statens Serum Institut in Copenhagen, Denmark, says, “Because older people are often more likely to experience serious disease symptoms and unfortunately die, our results make clear how important it is to introduce policies to protect the elderly during the pandemic.”
“Given the stakes,” he continues, “the findings highlight the importance of people adhering to measures put in place to keep themselves and others protected, even though they have already had COVID-19.”
Scientists have discovered that the immune systems of the elderly are less responsive to diseases and vaccines. This is known as immunosenescence.
Mass testing
Denmark provided free and reasonably fast PCR testing to anyone who needed it early in the pandemic, regardless of their symptoms.
As a result, a total of 10.6 million examinations were administered to 4 million individuals (more than two-thirds of the population) in 2020.
This helped researchers to find out what percentage of people who tested positive during the first wave of infections (from March 1 to May 31, 2020) would test positive again during the second wave (September 1 to December 31, 2020).
Just 72 (0.65 percent) of the 11,068 people who tested positive in the first wave tested positive again in the second.
After taking into account other factors, the researchers discovered that this equated to an overall defense against repeat infection of 80.5 percent across all age groups.
The researchers extended their scope in a second study to include first infections and reinfections that occurred at any time during the year. A similar level of overall security of 78.8% was verified.
A first infection, on the other hand, offered just 47.1 percent protection against a second infection in those aged 65 and up.
On a more positive note, overall levels of immunity against reinfection appeared to persist for at least 6 months.
“The closely related coronaviruses SARS and MERS have both been shown to confer immune protection against reinfection lasting up to 3 years, but ongoing analysis of COVID-19 is needed to understand its long-term effects on [people’s] chances of [catching the infection] again,” says study co-author Daniela Michlmayr, Ph.D., who is also from the Statens Serum Institut.
The study’s findings have now been published in The Lancet.
Cause for concern
Prof. Rosemary J. Boyton and Daniel M. Altmann of Imperial College London in the United Kingdom write in an accompanying comment articleTrusted Source that many people would find the study’s findings “relatively alarming.”
“Only 80% protection from reinfection in general, falling to 47% in people 65 and older are more worrying statistics than previous studies have offered,” they write.
They come to this conclusion:
“If proof were required, these findings show that protective immunity from natural infections might not be possible for SARS-CoV-2, and that a global vaccination program using high-efficacy vaccines is the only long-term solution.”
The new study’s authors admit that their analysis has certain limitations.
Most of the PCR tests, for example, were performed prior to the advent of new, more transmissible strains of the virus. These strains could be more adept at evading established immunity, increasing the risk of reinfection.
People who know they’ve already had the virus could be less likely to pursue testing again and to take precautions like wearing a face mask, according to the researchers.
When the researchers refocused their study on healthcare professionals, who have a high risk of infection but are more likely to obey infection prevention protocols, they found that they were protected from reinfection 81.1 percent of the time. This is almost identical to the general population.
There’s also a chance that if a person tests positive, the virus will stay in their body for months. This could account for some of the second positive tests.
When the researchers confined their comparison to experiments performed at least 4 months apart between the first and second waves of the pandemic, the apparent security against reinfection remained at 82.8 percent.
Importantly, the research does not address whether a person’s first infection protects them from a serious second infection (as opposed to one with few or no symptoms).
People’s test results are typically the only item reported in the Danish registry, not their symptoms.