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People who suffer mild Covid-19 symptoms may carry protective antibodies for only a matter of weeks, potentially complicating the search for a vaccine, a study suggests.
Researchers in Spain who screened nearly 70,000 people found that 14 per cent who were positive for antibodies in a first round of testing gave a negative result two months later. The apparent disappearance of antibodies was mostly seen in those who had very mild symptoms or who had been asymptomatic.
“Immunity can be incomplete, it can be transitory, it can last for just a short time and then disappear,” Raquel Yotti, director of Spain’s Carlos III Health Institute, which co-led the study, said. “We must keep protecting ourselves and protecting others.”
Other researchers said that the findings appeared to be in line with a tentative emerging consensus: that people who hardly notice that they have Covid-19 may not amass lasting antibodies, although other elements of the immune system may still protect them. “It fits the current picture,” Ian Jones, professor of virology at the University of Reading, said. “No symptoms suggests a mild infection, which never really gets the immune system going well enough to generate immunological ‘memory’.”
Professor Jones said that this raised two sets of concerns. “Anyone who tests positive by antibody test should not assume they are protected. They may be, but it is not clear,” he said.
Additionally, the vaccines undergoing trials will need to stimulate the immune system more vigorously than a mild real infection or they too could be short lived. “This is a nuisance as people may have to have regular boosters — and it might also provide material for the anti-vaccine lobby,” Professor Jones added.
The study also indicated that just 5.2 per cent of Spain’s population have developed antibodies against the disease, despite the country being one of the worst hit in Europe with roughly a quarter of a million recorded cases to date. A strict three-month lockdown from March 14 to June 21 helped reduce the rate of infections.
Health workers were twice as likely to have contracted coronavirus as the normal population. The report also found that over 30 per cent of those who had antibodies said that they had not felt ill despite being infected.
The report came as lockdowns were reimposed on more than 210,000 people in western Catalonia and 70,000 in the A Mariña region in Galicia due to some serious local outbreaks.
Professor Daniel Altmann, of the Department of Medicine at Imperial College London, said: “The Spanish study is sobering and confirms the picture from many other studies, both regional seroprevalence studies and longitudinal studies in recovered patients.
“The overall pool of antibody-positive people barely rises because as some gain immunity, others have lost theirs. It seems the nature of naturally induced immunity to SARS-CoV-2 is really quite a short-lived antibody response; the other key part of the immune response, T cells, may have memory that lasts several years but so far we lack the formal proof that they’re protective.”
Professor Altmann added: “The rather fragile, ephemeral immunity stimulated by natural infection is a specific feature of adaptations of the virus and its subversion of immunity. For example, interaction of the virus at first contact with host cells is meant to set off big alarm bells through the interferon system, but this is rather sub-optimal for SARS-CoV-2.
“Because this initial alarm is subdued, the subsequent immune response becomes somewhat altered. The job of good vaccine design is to bypass and overcome all these problems to stimulate a large, sustained, optimal, immune response in the way the virus failed to do.”