New studies are boosting assessments that immunity to COVID-19 lasts at least 6-8 months after recovery from the disease.
Research published in Science Immunology this week examined 25 patients recovering from the illness. Though antibodies — the immune system proteins that attack virus particles — began dropping in blood samples some 20 days after symptoms appeared, memory B cells that produce antibodies continued to rise in the blood for 150 days and remained high until the 240-day point. This signals subjects’ bodies were primed to fight off the virus for some eight months.
Meanwhile, researchers in two other studies found that people who made antibodies to the coronavirus were much less likely to test positive again for up to six months and maybe longer.
The results bode well for vaccines, which provoke the immune system to make antibodies.
A study published Wednesday by the New England Journal of Medicine involved more than 12,500 health workers at Oxford University Hospitals in the United Kingdom. Among the 1,265 who had coronavirus antibodies at the outset, only two had positive results on tests to detect active infection in the following six months and neither developed symptoms.
That contrasts with the 11,364 workers who initially did not have antibodies; 223 of them tested positive for infection in the roughly six months that followed.
A third study by the National Cancer Institute study involved more than 3 million people who had antibody tests from two private labs in the United States. Only 0.3% of those who initially had antibodies later tested positive for the coronavirus, compared with 3% of those who lacked such antibodies.
The results showed that people with antibodies from natural infections were “at much lower risk… on the order of the same kind of protection you’d get from an effective vaccine,” of getting the virus again, said Dr. Ned Sharpless, director of the US National Cancer Institute.
“It’s very, very rare” to get reinfected, he said.
The institute’s study had nothing to do with cancer — many federal researchers have shifted to coronavirus work because of the pandemic.
“It’s very gratifying” to see that the Oxford researchers saw the same risk reduction — 10 times less likely to have a second infection if antibodies were present, Sharpless said.
His institute’s report was posted on a website scientists use to share research and is under review at a major medical journal.
The findings are “not a surprise … but it’s really reassuring because it tells 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 had no role in either study.
“We don’t know how long-lasting this immunity is,” Wolf added. Cases of people getting COVID-19 more than once have been confirmed, so “people still need to protect themselves and others by preventing reinfection.”