Scientists are begging rich countries to halt their COVID-19 vaccine booster campaigns, saying they could be prolonging the pandemic by not donating them to countries with minimal coverage.
A widespread outbreak in a country with low vaccination rates would increase the chances of a variant that could render the current vaccines ineffective. But Israel - an early adopter of the highly effective Pfizer-BioNTech vaccine - has already started giving people a third dose, and a number of other countries are set to follow.
Their argument is the antibodies generated by the usual two-dose course of the current vaccines wane significantly after six months, so those who got their jabs early in the rollout need another dose now to keep up their protection against the Delta variant, which is better at infecting vaccinated people than the original strain of the virus.
Early data from Israel suggests it's working, with infection rates dropping amongst those who've had the third dose. It's not clear yet how long this increased level of protection lasts.
But the typical two-dose regime already offers significant protection, a new paper by an international group of scientists published in medical journal The Lancet on Tuesday, and the benefit of a third dose is minimal compared to the first and second.
"Taken as a whole, the currently available studies do not provide credible evidence of substantially declining protection against severe disease, which is the primary goal of vaccination," said lead author Ana-Maria Henao-Restrepo, an epidemiologist at the World Health Organization (WHO).
"The limited supply of these vaccines will save the most lives if made available to people who are at appreciable risk of serious disease and have not yet received any vaccine."
The WHO has been saying as much for weeks now, but the new study also included input from scientists at the US' federal government's Food and Drug Administration. The US is one of the nations planning a round of booster shots soon.
A number of poor nations across Africa, Asia and the Middle East have barely vaccinated anybody. CNN's global tracker, which uses figures collated by Our World in Data, says Congo, Haiti, Turkmenistan, Tanzania, South Sudan, Chad, Madagascar, Cameroon, Papua New Guinea, Somalia, Niger and others haven't even vaccinated 1 percent of their populations yet.
Despite antibody levels falling significantly six months after a person's second dose, the research says that doesn't necessarily mean the vaccines will eventually wear off. Antibodies are just one part of the body's immune system, and "memory responses and cell-mediated immunity… are generally longer-lived".
While variants like Delta can infect vaccinated people, the current vaccines remain very effective against severe disease - 95 percent across the studies looked at in the new review - and unvaccinated people "are still the major drivers of transmission and are themselves at the highest risk of serious disease".
"If new variants that can escape the current vaccines are going to evolve, they are most likely to do so from strains that had already become widely prevalent."
The paper says immunocompromised people might see greater benefit from a booster than most, it is "not known whether such immunocompromised individuals would receive more benefit from an additional dose of the same vaccine". Booster shots designed to fight the new variants - like we get every year in the flu vaccine - haven't been developed yet. Pfizer said in July it was working on one. Until then, the boosters being offered are exactly the same as those people have received before.
"The limited supply of these vaccines will save the most lives if made available to people who are at appreciable risk of serious disease and have not yet received any vaccine. Even if some gain can ultimately be obtained from boosting, it will not outweigh the benefits of providing initial protection to the unvaccinated," the paper concludes.
"If vaccines are deployed where they would do the most good, they could hasten the end of the pandemic by inhibiting further evolution of variants."
New Zealand purchased enough Pfizer doses to fully vaccinate everyone in the country, though the deliveries have fallen behind growing demand, forcing the Government to cut deals with other nations to get some earlier than its contract with the pharma giant allowed.
COVID-19 Response Minister Chris Hipkins last week said doses of a different vaccine - one made by Novavax - are expected to arrive early next year, subject to approval, and may be used in a booster campaign.