The US government aims to begin offering Covid-19 vaccine booster shots next week to Americans aged 16 and up. A panel of outside advisers to the Food and Drug Administration (FDA) voted against such broad coverage on Friday but backed a shot for people 65 and older, which the agency may or may not follow.

President Joe Biden's administration faces criticism within the scientific community over whether the additional shots are needed for the general population.

What does the data say about the need for boosters?

Pfizer Inc and its German partner BioNTech SE have each presented analyses of clinical trial data showing that the effectiveness of their shots, initially estimated at over 90 per cent against symptomatic Covid-19 infection, wanes over time.

As a result, they say, people who were vaccinated earlier on in the pandemic are now more vulnerable to so-called breakthrough infections, particularly in the face of the fast-spreading Delta variant of the coronavirus. Booster doses help restore the waning levels of antibodies produced by the original inoculation, the drugmakers have said.

Read: Breakthrough infections and vaccine efficacy: Why are vaccinated people still getting the virus?

Data from Pfizer's clinical trial of 30,000 people suggests that the vaccine's efficacy diminished by around 6pc every two months after the second dose. In a second trial testing booster shots in 300 participants, a third dose generated a better immune response than the second, according to Pfizer.

Moderna this week said it found higher rates of breakthrough infection among participants in its clinical trial who received its two-dose vaccine roughly 13 months ago compared with those vaccinated roughly eight months ago. The study period was from July and August when the highly contagious Delta variant was the predominant strain.

US health regulators are also closely watching real-world data on vaccine effectiveness and boosters from Israel, where a majority of the population has received the Pfizer/BioNTech vaccine. Israel is offering booster shots to everyone age d12 and up.

Israel's Health Ministry this week reported data showing that more than 1.1 million people aged 60 and older received a booster dose of Pfizer, resulting in a decline in overall infections as well as severe illness from Covid-19 in that group. The authors said their study suggests that a booster dose can restore vaccine efficacy among booster recipients to 95pc.

In addition to preventing severe illness, some health officials in the United States and Israel say booster doses may help reduce transmission of the coronavirus at a time of rising infection rates in many countries.

What is the critique?

Many vaccine experts say the data so far only suggests a need for boosters in older adults and people with compromised immune systems. The critics include two FDA scientists who resigned as the United States government announced its booster shot plans.

For example, a report from Israel's Health Ministry shows a much higher risk of severe breakthrough infection among vaccinated people over the age of 60 compared with people aged 50-59 and 40-49, even though the younger groups include people who were vaccinated more than six months ago.

Other studies in the United States show that vaccines remain highly protective against severe disease and death in adults under the age of 65.

Several other countries, including the UK, France and Germany, have so far limited booster plans to older adults and other high-risk groups.

The World Health Organisation has repeatedly implored the United States and other wealthy countries to hold off on offering booster shots and to use those doses to help inoculate the many people worldwide who have yet to receive their first doses.

Scientists also say that there are confounding factors in analysing waning vaccine effectiveness and the impact of boosters, including whether Delta may have a harsher impact on older adults and whether a return to mask-wearing in some places, rather than booster shots, is helping contain the spread.

They also say that randomised trials provide the best estimates of effectiveness in the real world, but most of the data so far is from observational studies or from small clinical trials.

Header image: A man is administered a dose of Covid-19 vaccine at a temporary vaccination centre set up in the cafe “Le Diplomate” in the Fives district of Lille, northern France, on September 18, 2021. — AFP