New data from the PITCH study has shed new light on how well immune protection from COVID-19 lasts over time, and the value of booster vaccine doses in maintaining protection against infection.
Both infection and vaccination, alone or in combination, generate antibody and T cell responses against COVID-19. However, the duration of such responses – and resulting protection from disease – depends on a wide range of factors.
The PITCH (Protective immunity from T cells in Healthcare workers) study is a large prospective study of UK healthcare workers, and is part of the wider SIREN study. Results from SIREN have previously shown that prior infection with COVID-19 has a substantial impact on the immunity given by vaccination.
In this study, published as a preprint and therefore not yet peer reviewed, the researchers looked at immunity over longer periods of time (6-9 months) in 684 healthcare workers, following two doses of the Pfizer or AstraZeneca vaccine, and up to six months following a subsequent mRNA booster vaccine dose.
They found differences in the dynamics of the two distinct types of immunity (humoral and cellular). Binding and neutralising antibodies waned over the six months following a second dose, whereas T and memory B cell responses were maintained after the second vaccine dose.
They also found that a booster vaccine dose broadened a person’s ability to neutralise variants of concern including Omicron (BA.1, BA.2 and BA.5), and also boosted T cells to levels greater than those seen six months after a second dose.
People who had previously been infected with COVID-19 had broader T cell responses compared with those who had never been infected, and this was still true six months after a third dose.
Overall, T cell responses were well maintained over time – especially in those who had been both vaccinated and infected. The researchers deduced from this that T cells likely play a key role in maintaining protection against severe disease.
The researchers concluded from their results that a third dose of vaccine confers a measurable benefit, including in people who have previously been infected, and irrespective of the type of vaccine originally given.