A recent study at University College London looked at a cohort of 129 healthcare workers at high risk of an infection for 16 weeks. Of these, 57 never tested positive for the virus. The scientists discovered the healthy workers had mounted a “robust” T-cell response to Covid.
T-cells are part of the body’s adaptive immune system – they remember past infections. The researchers concluded that T-cells that had encountered common colds can also recognise Covid. “The evidence points to protection originating from prior exposure to endemic common cold coronaviruses,” said author Francois Balloux, professor of computational systems biology at UCL. However, he also suggested that they may have previously had Covid itself without knowing it: “Latent infection to low levels of SARS-CoV-2 might also have played a role.”
Research by Public Health England, published last year, found one in four healthcare workers had high levels of T cells which recognised Covid, suggesting they had some protection against the virus, but nearly half had never been infected. “About half the people with high levels of T-cells in their blood have not had Covid-19, as far as we could tell – the cells were probably there because of previous infection with coronaviruses other than SARS-CoV-2,” the authors said.
Danny Altmann, a professor of immunology at Imperial College London, says: “The UCL study asked whether there is any immune signature that may be present in people who are clearly exposed to the virus yet never seen to become PCR or antibody positive. Could there be some kind of innate immune pathway which for some people heads the virus off before it ever gets established? The paper shows evidence for this kind of response in some, but the problem is you can’t easily tell if you have it and it certainly wouldn’t be an alternative to getting vaccinated.”
Previous studies have found that vaccination after infection stimulates an antibody response that’s more vigorous and probably more lasting than either recovery or vaccination alone. Studies indicate that natural immunity will last for around eight months. After about a year, the immune system may become more vulnerable to variants.
However, vaccination very effectively boosts the immune system’s memory of the coronavirus. In one study researchers found that people who were both previously infected and vaccinated developed 100 times the protective antibodies against the beta variant compared to those who only were infected.
And a recent report published by the US Centers for Disease Control and Prevention concluded that people vaccinated after catching Covid have half the risk of reinfection compared with previously infected people who haven’t been vaccinated at all. The effect is so powerful that several articles concluded that one dose is enough to protect those who have previously had Covid and made antibodies. France, Germany and Italy are among the countries that advise just one dose for these people.
Eleanor Riley, a professor of immunology and infectious disease at the University of Edinburgh, says: “It’s not unusual for infection to provide broader protection than vaccination. The vaccine contains the spike protein, which is only one of more than 20 proteins that the virus makes. Potentially this means that previously infected people may have immune systems that recognise and mobilise against more of the whole virus, including parts that are less liable to mutations than the spike. This could protect you against future variants.”
— to www.telegraph.co.uk