Most adults would struggle if presented with an exam paper from their secondary school days. Since those teenage years, memories of formulas and facts that were once quick to recall have likely faded.
It’s the same with vaccines, which can be thought of as ‘lessons’ for the immune system. Over time, a booster is needed to remind the body how to defend itself from viruses, especially when new information (in the form of variants) arises.
Protection derived from COVID-19 vaccines typically wanes around six months after the initial shot. Some studies suggest that those inoculated with a different booster than their first doses—known as a heterologous vaccination—may develop better antibody responses than those who received the same follow-up shot.
Antibodies are just one part of the immune system’s arsenal against viruses. “Immune responses are very complex and work as a big network,” explained Angeline Rouers, Senior Research Fellow at A*STAR Infectious Diseases Labs (ID Labs), who was interested in how boosters prime the cellular component of the immune system.
To answer this question, Rouers and colleagues worked with teams from A*STAR’s Singapore Immunology Network (SIgN); the National University of Singapore; the National Centre for Infectious Diseases, Singapore; Tan Tock Seng Hospital; and Nanyang Technological University to enlist 79 patients who had received a Moderna booster following two doses of the Pfizer-BioNTech (BNT162b2) vaccine.
The participants’ immune responses (including antibody levels, memory B cell and T cell responses) were measured immediately, seven days and 28 days after vaccination and were compared against those who had received three consecutive BNT162b2 shots.
Results from the team’s experiments lined up with previous reports that showed an uptick in neutralising antibody levels following heterologous vaccinations. Using a specialised assay called B cell ELISpot, the researchers explored the immune response more deeply and found that regardless of the regimen, booster shots also triggered an increase in memory B cells after 28 days.
According to Rouers, memory B cells work like antibody factories in case of re-exposure and can indicate of the person’s level of immune protection. “Memory B cells are responsible for antibody production when the immune system is recalled,” said Rouers.
“It is crucial to assess their presence and number to estimate the longevity of the immune response.”
Notably, compared to homologous vaccinations, heterologous ones led to significantly stronger memory B cell responses in adults aged over 60 years roughly 1 month after their third dose. This finding suggests that for seniors who are among those at risk of severe COVID-19 and death, recommending heterologous vaccinations might be a good public health strategy to keep them safe.
“These data will be helpful to guide policymaking and assist healthcare workers in deciding how to best immunise the vulnerable population,” Rouers added.
The team is expanding this work by testing different combinations of heterologous vaccinations as part of their mission to elucidate immune dynamics following COVID-19 boosters.