VACCINE-MAKERS HAVE developed covid-19 jabs at unprecedented speed. Now they are turning their attention to booster vaccines. In February Pfizer/BioNTech, the makers of one jab, said they had started a clinical trial looking at giving another shot to people who had already received two doses six to 12 months earlier. This month preliminary data released by Moderna, an American drugmaker, showed that a further dose of one of two jabs, tailored to new variants of the virus and given six to eight months after the first round of vaccinations, triggered a strong immune response against variants first identified in South Africa and Brazil. And this week the Financial Times reported that the vaccine developed by Oxford University and AstraZeneca is effective when used as a booster shot, according to the results of an upcoming study. But some experts argue that there are not yet enough data to determine whether booster shots are even needed. Why might the world require another shot in the arm, and how would booster shots work?
Booster shots are supposed to solve two problems: that immunity tends to wane after vaccination or infection; and that variants might be better at evading the immune system’s defences. Covid-19 vaccines carry out a mock attack on the body for the immune system to practise on. Once the infectious pathogen has been fought off, specialised “memory” B and T cells will stand ready to combat any reinfection. A booster shot is designed to remind the immune system how it responded to a previous infection or teach it to respond to an evolved threat, increasing the level of protection. It could use the vaccine a person received the first time or a slightly modified version.
Pfizer has said that people are likely to need a booster dose a year after their initial vaccination; Moderna thinks the same, for as long as the virus is circulating widely. But there is little evidence so far to confirm this. Although covid-19 vaccines have been tested in clinical trials, the duration of the protection they confer in the real world is still unknown. To determine this, scientists need to observe immunity levels in vaccinated people over many months. “Until that’s been looked at, we don’t know if there’s a problem that needs to be solved by further boosts next autumn or winter,” says Danny Altmann, a professor of immunology at Imperial College London. From June a trial in Britain will test how an additional dose of one of several vaccines affects the immune responses of people who had their first doses in December or January.
It is also unclear whether new variants of the virus will evade existing jabs. Most vaccines tested in clinical trials so far prevent most symptomatic and almost all severe cases of covid-19. They also appear to offer sufficient protection against the known variants of the virus to prevent severe disease and death.
However different variants pose different threats. “We need to understand the nature of each variant, and the immune responses required to generate a level of protection before we can decide if any tweaks are needed to the next round of booster vaccinations,” explains Sarah Gilbert, one of the scientists behind the Oxford/AstraZeneca vaccine. If needed, vaccine-makers could produce shots tailored to different variants, or a “bivalent” vaccine that would work against the original strain and a mutant. Pfizer/BioNTech, for example, have said that they can create a jab adapted to new variants in six weeks. In the meantime covid-19 will continue to circulate and evolve. It is still too early to tell whether a single regimen of vaccines is enough to bring the disease under control, or whether the world will need a boost.