Two new studies provide encouraging evidence that having COVID-19 may offer some protection against future infections. The researchers found that people who produced antibodies to the coronavirus were much less likely to test positive again for up to six months or more.
The results bode well for vaccines, which cause the immune system to produce antibodies – substances that bind to a virus and help eliminate it.
The researchers found that people with antibodies to natural infections were “at a much lower risk … in the order of the same type of protection you would get from an effective vaccine”, of getting the virus again, said Dr. Ned Sharpless, director of the US National Cancer Institute.
“It is very, very rare” to be infected again, he said.
The institute’s study had nothing to do with cancer – many federal researchers began to work with the coronavirus because of the pandemic.
Both studies used two types of tests. One is a blood test for antibodies, which can last for many months after infection. The other type of test uses nasal samples or other samples to detect the virus itself or pieces of it, suggesting current or recent infection.
A study, published on Wednesday by the New England Journal of Medicine, involved more than 12,500 healthcare professionals in hospitals at Oxford University in the United Kingdom. Among the 1,265 who had antibodies to the coronavirus at baseline, only two tested positive for active infection in the next six months and none developed symptoms.
This contrasts with the 11,364 workers who initially had no antibodies; 223 of them tested positive for infection in the next six months or so.
The National Cancer Institute study involved more than 3 million people who tested for antibodies in two private laboratories in the United States. Only 0.3% of those who initially had antibodies subsequently tested positive for coronavirus, compared with 3% of those who did not have these antibodies.
“It’s very gratifying” to see that Oxford researchers observed the same risk reduction – 10 times less likely to have a second infection if antibodies were present, Sharpless said.
His institute’s report was published on a website that scientists use to share research and is being analyzed by a leading medical publication.
The findings “are not a surprise … but they are really comforting because they tell people that immunity to the virus is common,” said Joshua Wolf, an infectious disease specialist at St. Jude Children’s Research Hospital in Memphis, who has not participated in any of the two study.
The antibodies themselves may not be providing protection, but they may just be a sign that other parts of the immune system, such as T cells, are able to fight any new exposure to the virus, he said.
“We don’t know the length of that immunity,” added Wolf. Cases of people receiving COVID-19 more than once have been confirmed, so “people still need to protect themselves and others by preventing reinfection”.