There is “almost certainly no urgency” to press ahead with booster shots for healthy adults and it may be better to see how the pandemic pans out before a decision is made, the scientist leading key research into third shots has said. Prof Saul Faust, chief investigator of the Cov-Boost study whose data next week is expected to help inform a decision on the rollout of boosters across the UK, told the Guardian that for now it may be preferable to prioritise only the vulnerable, including those with compromised immune systems. Meanwhile, some scientists said booster shots may be useful for routine use even among highly vaccinated populations to reduce Covid transmission, especially given the prevalence of the Delta variant. On Thursday, scientists including Prof Neil Ferguson suggested that even if evidence did not yet show waning protection in the double-vaccinated against serious illness and death, booster shots could help reduce the spread of cases. On the recommendation of the Joint Committee on Vaccination and Immunisation (JCVI), it was announced on Wednesday that half a million people aged 12 and over with severely weakened immune systems are to be offered third Covid vaccine shots. A decision on whether to offer boosters for all healthy adults was expected to be announced next week at the earliest, the Guardian understands, although it could take longer. The Cov-Boost study – which is designed to assess the use of one of seven different vaccines when given as a third dose, on top of the two doses of either the Pfizer/BioNTech or Oxford/AstraZeneca vaccines – is understood to be a key piece of research to inform the JCVI decision. Faust said the study would help answer which vaccines were most effective as a third shot, depending on what jabs were given as the initial two shots, rather than which populations should receive a third dose. The committee will have access to early study findings some time next week, he added. Last week, there were an average of more than 33,750 daily cases across the UK compared to less than 1,400 at the same time last year – before the rise of the highly infectious Delta variant and before all restrictions were eased – and 61,000 at the peak in January. But with nearly 80% of UK adults having received two vaccine doses, hospitalisations and deaths are far lower than at their peak. High numbers of cases increase the risk to unvaccinated or otherwise vulnerable people, and could lead to the emergence of new variants. Many rich countries with strong vaccine coverage have announced the routine rollout of third shots amid concerns about waning immunity. Preliminary data shows that people experience a fall in antibody levels weeks and months after getting their jabs, but so far the fall does not appear to have reduced protection against severe disease and death. In general, there is little scientific consensus on what level of antibodies and other tools in the immune system’s armament confer protective immunity. However, boosters might work to prevent more infections and transmission, suggested Helen Petousis-Harris, a vaccinologist from the University of Auckland. “Because in order to prevent that infection, we really need the presence of these antibodies, and if they’ve waned, that might make establishing an infection easier.” UK data suggests somebody who is double-vaccinated is about half as likely to be infected as someone who is unvaccinated, and although the double-vaccinated person is far less likely to develop severe disease, they can still infect others. “The other thing is, boosters could help improve immunity to variants, and people who might not have responded so well at the beginning to the primary doses might also benefit from a booster,” Petousis-Harris added. “It’s likely that these things will contribute to the overall community immunity.” Israel, for instance, has already kicked off its booster programme. “I think in Israel, they actually want to suppress transmission … If you have even 10% of your population unvaccinated, then one way of reducing risk to them is by boosting the immunity of everyone else,” said Ferguson, an epidemiologist at Imperial College London. “I think the UK is adopting a much more incremental approach – so at the moment our joint committee on vaccination is only recommending boosters [third shots] for clinically extremely vulnerable people, which is only about 1% of the population.” Booster shots for those with weak immune systems, who may not have mounted a good response to the first two jabs, is a way to keep the most vulnerable safe, the head of the World Health Organization, Hans Kluge, suggested earlier this week. However, many scientists have also underscored that rich nations should not hoard vaccine supplies for third shots while some poorer countries have not even been able to administer first shots to the most vulnerable.