By Gretchen Vogel

As the extraordinarily infectious Delta variant of SARS-CoV-2 continues to spread around the world, vaccines’ powers are showing their limits. Although they are still extremely effective at preventing severe COVID-19, the tantalizing hope that the shots could block almost all infections—and squelch transmission—has evaporated. That has upended return to office and school plans, threatened economic recoveries, and spurred fresh political rows over mask and vaccination mandates.

Now, amid hints that vaccine-induced immunity is waning, policymakers and scientists are debating whether widespread booster shots could help—or whether getting shots into the arms of the unvaccinated should remain the top priority. And many people wonder whether one booster will suffice or periodic COVID-19 vaccination will become the new normal, as it is for influenza.

On the latter question, some scientists say experience with other vaccines suggests a single well-timed booster may provide long-lasting immunity. But others contend the booster rush is premature given scant data on their effectiveness and best timing. “We don’t understand who is going to need a booster, how long after their last dose, or which vaccine combination works best,” says physician-epidemiologist Bruce Aylward, a senior adviser at the World Health Organization (WHO). “You need to understand all that before you decide how boosters should be used.”

That hasn’t stopped booster rollouts in countries like Israel, which is seeing a Delta-fueled surge of COVID-19 cases, hospitalizations, and deaths, despite one of the earliest and fastest vaccine campaigns in the world. More than 60% of the Israeli population has received two doses of Pfizer’s messenger RNA (mRNA) vaccine, but on 30 July, Israel began to offer a third dose of the vaccine to anyone 60 and older—the first country to do so. On 20 August, it said everyone 40 and older should get one.

WHO and other organizations have warned strongly against such broad booster rollouts, mainly because many high-risk people worldwide have not even received a first vaccine dose. Giving boosters now “is unfair to say the least, potentially … even criminal,” says Tulio de Oliveira, a computational biologist at the University of KwaZulu-Natal, Durban, who has used sequencing to track the pandemic’s spread in Africa.

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