How long does Covid immunity really last?
vaccinated after infection causes the number and range of memory B cells (immune cells that can spring into action to produce Covid antibodies) to expand dramatically which very likely protects people against the dreaded variants. The study participants had received mRNA vaccines as the AstraZeneca jab isnt yet approved in the US. Eleanor Riley, a professor of immunology and infectious disease at the University of Edinburgh, says the study shows that: Antibodies improve in efficacy over time. We may have fewer antibodies but the ones we have are better and more effective. It appears that its a case of survival of the fittest as the not very good antibodies die out and the good ones persist. Whats more, she says, antibodies are very efficiently boosted by a single vaccination. All in all, she says, the news is very reassuring and suggests that immunity to Covid-19 lasts longer than we might have feared a few months ago. Prof Riley adds that even if you were infected with the original strain, you are likely to be at least somewhat immune to being reinfected with Delta. This is because exposure to the whole virus means that people tend to develop immune responses to all its components, making it much harder for the virus to evade your immune system. Maybe not, she concedes. Almost certainly yes.  Based on these findings, plus the existing knowledge that vaccines boost antibodies even more than infection, researchers suggested that immunity might last for years, possibly a lifetime, in people who were infected with the coronavirus and later vaccinated. But it was unclear whether vaccination alone might have a similarly long-lasting effect. We cant be entirely sure, admits Prof Riley, but she says, the response is likely to be similar. More certainty is on the way. Participants in the first vaccine trials are now getting called back for their one-year follow-up so, she says, we should find out how well their immune response is faring against the variants in the next few weeks. There is yet more good news when it comes to mixing and matching vaccines. Research involving 800 people showed that giving different vaccines could be even more effective than double-dosing with two of the same. When people were given a combination of the AstraZeneca and Pfizer vaccines, the antibody boost was even greater than from two doses of the Oxford vaccine, though not from two Pfizer jabs. However, mixing vaccines created a more powerful T-cell response than using two shots of either vaccine on their own. T-cells are immune cells that can target and destroy virus-infected cells and provide a back-up to antibodies. [It] seems less urgent than we thought three months ago, says Prof Riley. Certainly, the data indicates that most people who have been vaccinated up to now can be confident about their level of protection for a while yet.  Michel Nussenzweig, an immunologist at Rockefeller University in New York who led the BioRxiv study, told The New York Times that his research indicates that fully vaccinated people who previously recovered from Covid may not need a booster shot at all, possibly ever. But those who are vaccinated may require one at some point. Its very uncommon not to make an antibody response to vaccination, says Prof Riley. Even if you didnt make one to the first dose, after the second we see pretty much everyone coming back positive. However, she warns: There are always a few people for whom vaccination doesnt work as well. These can include people on chemotherapy and immunosuppressant therapies and the frail elderly. They may be first in line for boosters.  Tim Spector, a professor of genetic epidemiology at Kings College London and creator of the Zoe Covid Symptom Study app, says that his data suggests a wide range of duration of immunity. In older people with health problems or obesity it could be as low as five months, he says. We expect to see some older people who were vaccinated in December and January starting to lose immunity in the next few months.  If they do, then they can take heart from yet another study which shows that an additional shot of the Oxford/AstraZeneca vaccine, given more than six months after the second, leads to higher antibody responses against the Alpha, Beta and Delta variants, first identified in Kent, South Africa and India respectively. Daniel Altmann, a professor of immunology at Imperial College London, points to a US study from May which looked at the link between antibody levels and immunity. It suggests that people should be protected from severe infection even if their antibodies have fallen six-fold from the level needed to prevent any symptomatic infection. Prof Altmann says: Id bet my house on most people not seeing their vaccine-induced antibodies waning to a dangerous level within two years. But we need research with the largest possible dataset. This is what I want to know before we roll out boosters.