For now, COVID-19 booster shots are necessary because the number of antibodies in blood decreases over time. With mRNA vaccines, the efficacy appears to start waning about six months after people receive a second dose.

With one-shot vaccines such as the one developed by Johnson & Johnson, the German Standing Committee on Vaccination (STIKO) even recommends that people get a booster shot before the six months are up.

Future COVID-19 vaccines will probably have to be adjusted to protect effectively against new variants of the coronavirus, just as flu shots are adjusted to new strains. Vaccines are already being developed to protect against mutations of the delta variant.

Facing endemic COVID-19

Given the current rates of infection in Europe, there is still a chance that herd immunity could be achieved through a combination of contagion and vaccinations.

When it comes to immunity, it might not only be a question of antibodies, as indicated by the preliminary, not-yet-peer-reviewed findings of a study conducted by a large team of researchers from Britain and Singapore that was recently published in the scientific journal Nature.

Over a period of months, the researchers monitored health care workers who had potentially been exposed to the coronavirus but not become recognizably ill from COVID-19 and never tested positive. Serological antibody tests did not show any notable results either. 

Robust memory T-cells

The researchers found that the 58 seronegative health care workers (SN-HCW) had more multispecific memory T-cells than a comparative cohort whose potential exposure to the coronavirus was much lower.

The T-cells were directed in particular against the replication transcription complex (RTC), which effectively spreads the virus.

The study found that the T-cells of the SN-HCWs had a higher amount of IFI27, a protein that is "a robust early innate signature of SARS-CoV-2," and concluded that this was a suggestion of "abortive infection."

Therefore, the T-cells had possibly interrupted infection early on. What remains unclear is where the 58 SN-HCWs got their unusually high T-cell immunity from. Could it have come from a prior infection with a different coronavirus, for example a cold virus?

A possible conclusion could be that the repeated exposure to coronaviruses, such as SARS-CoV-2 — should it become endemic and should people come into frequent contact with a small number of pathogens — could lead to immune systems' being better able to cope, either with antibodies or T-cells. This would bring us one step closer to herd immunity.

So far, the researchers have recommended caution and insist that nobody should feel entirely safe and assume that they are immune to the coronavirus because there is still a very high risk of not being immune.