A team of scientists, including some from the Danish Department of Epidemiology and Infectious Disease Prevention, noted that most people who had Covid-19 appeared to have protection against reinfection for about six months. But a check of the demographics of those who were being infected again showed that they were mostly people aged 65 and over.
They analyzed the rate of reinfection among 4 million people during the second wave of Covid-19 from September to December 31 and compared it with the rate of infection during the first wave between March and May. Of the 11,068 people who tested positive during the first wave, only 72 tested positive again during the second wave.
The older age group had only about 47% protection against repeated infections, compared to younger people who appeared to have about 80% protection against reinfection, the team wrote. The discovery is not entirely unexpected, as as people age, their immune system weakens.
“Given what is at stake, the results emphasize the importance of people adhering to the measures implemented to keep themselves and others safe, even if they have already had COVID-19,” co-author of the study Dr. Steen Ethelberg of Statens Serum Institut in Denmark said in a statement.
“This is a marked difference,” said Dr. Amy Edwards, an infectious disease specialist at University Hospitals in Cleveland, who was not involved in the study.
“I think it really emphasizes the importance of ensuring that we vaccinate everyone over 60, whether they are Covid or not, to protect them from future infections.”
In a commentary accompanying the study, immunologists Dr. Rosemary Boyton and Daniel Altmann of Imperial College London called the difference in the rate of reinfection “relatively alarming.”
“Only 80% protection against reinfection in general, decreasing to 47% in people aged 65 and over, are more worrying numbers than those offered by previous studies,” they wrote. SARS-CoV-2 the hope of protective immunity through natural infections may not be within reach and a global vaccination program with highly effective vaccines is the lasting solution. “
The researchers analyzed test data from Denmark involving 10.6 million coronavirus tests done by about 4 million people, or about 69% of the country’s population.
They analyzed rates of reinfection during the second wave of Covid-19 from September to December 31 and compared them with rates of infection during the first wave of infection between March and May. Of the 11,068 people who tested positive during the first wave, only 72 tested positive again during the second wave. This represents less than 1% of infected people.
But 3.6% of people aged 65 and over were infected again in the second wave.
This is not unexpected, due to what is known as immunosenescence – the gradual deterioration of the immune system that comes with age.
“There is a reason why people over 60 need to receive extra vaccines to increase their immunity to various infections, because we know that the immune system starts to decline later in life,” said Edwards.
One good thing about mRNA vaccines, like that of Pfizer and Moderna, Edwards said, is that the vaccines seem to overcome some of the concerns about immunosenescence because they produce robust protection.
“We don’t know yet whether people will need reinforcements or not, but it will be interesting to watch and see how it plays out,” said Edwards.
A limitation of the study is that it examined infections before there were many variants in circulation, so it is unclear what impact this could have on the rate of reinfection. This is something that scientists will have to look into in the future.