What are COVID-19 vaccine booster shots and why are they being offered?

The head of the World Health Organization (WHO) has called for countries to delay rolling out booster shots of COVID-19 vaccines to close the gap between richer and poorer nations.

Tedros Adhanom Ghebreyesus said: "I understand the concern of all governments to protect their people from the Delta variant. But we cannot accept countries that have already used most of the global supply of vaccines using even more of it.”

Several high-income countries have plans to offer a third, booster shot to vulnerable people, but the majority of people in low-income countries remain unvaccinated. WHO data shows just over 1% of those in low-income countries had been vaccinated as of August 4, compared with more than 50% of those in high-income countries.

The WHO wants a moratorium on boosters to help to achieve its goal of vaccinating 10% of every country’s population by the end of September.

Vaccine efficacy has been shown to reduce over time, as the virus-fighting antibodies it stimulates naturally wane. For example, Pfizer reported protection against symptomatic disease drops to 84% after six months, from a peak of 96% within two months of receiving the first two doses.

The company, which developed its vaccine with partner BioNTech, is seeking approval from US and European regulators for a third dose. "The Pfizer vaccine is highly active against the Delta variant," Pfizer's Chief Scientific Officer, Mikael Dolsten said. But after six months, "there likely is the risk of reinfection as antibodies, as predicted, wane".

Israel, which has used the Pfizer/BioNTech vaccine on 57% of its population, became the first country to begin a booster campaign - for the vulnerable and those aged over 60. President Isaac Herzog received his third dose of the vaccine at the end of July.

Protection against serious disease dropped among those vaccinated in January, according to preliminary studies in Israel.

Another advantage of a booster shot is that it can be modified to target specific variants, notes Yale Medicine.

However, the WHO’s vaccines chief Dr Katherine O’Brien, said the evidence around the need for a third shot was still “evolving”: “It’s moving. We don’t have a full set of evidence around whether this is needed or not.”

France, Germany and several countries in the Middle East have already begun booster programmes, while the US plans to offer booster shots to all Americans from September.

From early September, Britain plans to offer COVID-19 boosters to 32 million adults, starting with the most vulnerable, the Telegraph reports, depending on the advice of the independent Joint Committee on Vaccination and Immunisation (JCVI).

“The booster programme would aim to provide additional resilience against variants, and maximize protection in those who are the most vulnerable to serious disease from COVID-19 ahead of the winter months, when there is increased pressure on the NHS,” the government’s COVID-19 Response: Summer 2021 guidance notes.

Analysis from Public Health England found that just one dose of either the Oxford/AstraZeneca or Pfizer/BioNTech vaccine “reduces the risk of symptomatic disease with the Delta variant by 35%, and hospitalisations by 80%”. The second dose boosts protection to 79% against symptomatic disease and 96% against hospitalization.

The guidance also notes: “Over the longer term, booster vaccinations are likely to become a regular part of managing COVID-19, along with other pharmaceutical interventions including antivirals and therapeutics.”

In the US, the CDC recommends people who are "moderately to severely immunocompromised" should receive an additional dose of mRNA COVID-19 vaccine after the initial two doses.

Roughly 3% of the adult population are in that category and includes those who have: