New studies reinforce estimates that immunity to COVID-19 lasts for at least 6-8 months after recovery from the disease.
A study published this year in Science Immunology examined 25 patients recovering from the disease. Although antibodies – immune system proteins that attack viral particles – began to fall into blood samples some 20 days after the onset of symptoms, antibody-producing memory B cells continued to grow in the blood for 150 days and remained high until 240 days. This signals that the bodies of the subjects were prepared to fight 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 possibly longer.
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The results suggest vaccines that provoke the immune system to make antibodies.
Read: COVID-19 antibodies disappear quickly. This does not mean that mass reinfection is expected
The study, published Wednesday in the New England Journal of Medicine, involved more than 12,500 health workers at Oxford University Hospitals in the UK. Of the 1,265 who initially had antibodies to the coronavirus, only two tested positive for active infection in the next six months, and none developed symptoms.
This is in contrast to the 11,364 workers who initially had no antibodies; 223 of them had a positive infection test in the approximately six months that followed.
The third study by the National Cancer Institute involved more than 3 million people who had antibody tests from two private laboratories in the United States. Only 0.3% of those who initially had antibodies later tested positive for coronavirus, compared with 3% of those who did not lack such antibodies.
The results showed 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,” to re-infect the virus, said Dr. Ned Sharpless, director of the U.S. National Cancer Institute.
“It’s very, very rare” to get infected again, he said.
The institute’s study had nothing to do with cancer – many federal researchers switched to working on the coronavirus because of the pandemic.
“It is very gratifying to see that researchers from Oxford have noticed the same risk reduction – 10 times less likely to have another infection if antibodies are present,” Sharpless said.
The report of his institute is published on a website used by scientists to share research and is under review in a major medical journal.
The findings “aren’t a surprise … but they’re really reassuring because they tell people that immunity to the virus is common,” said Joshua Wolf, a infectious disease specialist at St. John’s Children’s Research Hospital. Jude in Memphis who had no role in any of the studies.
“We don’t know how long-lasting this immunity is,” Wolf added. The cases of people who have repeatedly received COVID-19 have been confirmed, so “people still need to protect themselves and others by preventing re-infection.”