New studies are increasing the 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 disease. Although antibodies – the immune system proteins that attack viral particles – started to drop in blood samples about 20 days after the onset of symptoms, the memory-producing B cells that produce antibodies continued to rise in the blood for 150 days and remained high until the point of 240 days. This signals that the individuals’ bodies have been prepared to fight the virus for about eight months.
Meanwhile, researchers in two other studies found that people who produced antibodies to the coronavirus were much less likely to test positive again for up to six months or more.
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The results bode well for vaccines, which cause the immune system to produce antibodies.
Read: COVID-19 antibodies disappear quickly. This does not mean that mass reinfection is approaching
A study published on Wednesday by the New England Journal of Medicine involved more than 12,500 health 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.
An image produced by US researchers shows particles of the SARS-CoV-2 virus (red), covering the human bronchial cilia (blue) and mucus (yellow). (Ehre Lab, UNC School of Medicine)
A third study by the National Cancer Institute involved more than 3 million people who had been 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.
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”, of catching 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.
“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 was not involved in none of the studies.
“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”.