NEW YORK (Reuters) – The U.S. government aims to begin offering COVID-19 vaccine booster shots widely next week to Americans age 16 and up. The panel of advisors outside the Food and Drug Administration rejected broad coverage but supported a shot for those 65 and over, which may or not be followed by the agency.

President Joe Biden’s administration is being criticized by the scientific community about whether additional shots are necessary for the whole population.

WHAT DO THE DATA TELL US ABOUT BOOSTERS NEED?

Pfizer Inc (NYSE:) and its German partner BioNTech SE (NASDAQ:) Moderna (NASDAQ:) Inc have each presented analyses of clinical trial data showing that the effectiveness of their shots, initially estimated at over 90% against symptomatic COVID-19 infection, wanes over time.

According to them, those who had been vaccinated before the outbreak are more susceptible to the so-called breakthrough diseases, especially in light of the rapid-growing Delta coronavirus. According to drugmakers, booster doses are used to restore the lost antibodies from the original vaccine.

Pfizer found that the vaccine’s effectiveness decreased by about 6% after two-months of clinical trials involving more than 30,000 patients. Pfizer found that a third dose produced a stronger immune response to the vaccine than did the first. This was based on 300 participants participating in another trial.

Moderna announced this week it has found a greater rate of breakthrough infected among the participants in its clinical trials who had received their two-dose vaccine around 13 months ago, compared to those who were given vaccinations about eight months ago. Study period covered July through August when highly contagious Delta strain was predominant.

U.S. regulators closely monitor real-world data about vaccine effectiveness, boosters and other information from Israel. The majority of Israelis have received the Pfizer/BioNTech vaccine. Everyone 12 years old and older can get booster shots from Israel.

The Israeli Health Ministry reported this week that over 1.1 million older people received booster shots from Pfizer. This resulted in an overall decline in infections and severe COVID-19-related illness in the group. According to the authors, booster doses can improve vaccine effectiveness by up to 95% among booster recipients.

Some health professionals in Israel and the United States believe booster doses can help prevent severe illnesses. They also may reduce the transmission of coronavirus in a period of high infection rates across many other countries.

HOW DO YOU IDENTIFY THE CRITERIA?

Many experts in vaccines believe that the evidence so far suggests that boosters are needed for older people and those with compromised immune systems. Two FDA scientists were among those who quit after the announcement of booster shots by the United States.

According to an Israeli Health Ministry report, there was a greater risk of severe breakout infection in people over 60 years old than those aged 50-59 or 40-49. The younger group includes people who had their vaccines more recently than six months.

Studies in the United States have shown that vaccines are still highly effective in protecting against serious disease and death among adults younger than 65.

Not only have the United States, France, Germany and Britain limited vaccine boosters to elderly people and those at high risk, but many other countries like Germany and France also limit their availability.

World Health Organization repeatedly urged the United States, and other rich countries, to not offer booster shots. Instead use the doses to inoculate those many who are still waiting for their doses.

Scientists believe there may be confounding factors when analyzing the decline in vaccine effectiveness and impact of boosters. For example, whether Delta has a more severe effect on elderly adults or if a return in mask-wearing is helping to contain spread.

The best evidence of efficacy in the real world comes from randomized trials, however most data has been gathered from observational studies and small clinical trials.