The United States authorized the Pfizer and Moderna Covid vaccine boosters to all people aged 18 and older on Friday, as the world's hardest-hit country enters a new winter wave of the pandemic.

Boosters were previously available to the immune-compromised, over 65s, people at high risk of severe disease, and people in high-risk occupations.

The new decision "helps to provide continued protection against Covid-19, including the serious consequences that can occur, such as hospitalization and death," said acting FDA commissioner Janet Woodcock in a statement.

"This emergency use authorization comes at a critical time as we enter the winter months and face increasing Covid-19 case counts and hospitalizations across the country," added Moderna CEO Stephane Bancel.

The FDA said it based its decision on strong immune response data from hundreds of people dosed with the two vaccines. 

Pfizer also carried out a clinical trial involving 10,000 people aged over 16 which found that the booster showed an efficacy against symptomatic infection of more than 95 percent compared to those who did not receive a booster.

This study was not cited by the FDA in making its decision, but it could nevertheless indicate the Pfizer vaccine works best as a three-dose shot -- or that the three-week time interval between first and second dose was never long enough to induce the best immune response.

Both vaccines are available to people six months after completing their primary series. 

Pfizer's vaccine is dosed at 30 micrograms, the same as the primary series, while Moderna's is 50 micrograms, half the primary series.

People who received the Johnson & Johnson one-dose vaccine were already eligible for a booster two months after their first shot.

The booster decision comes as cases are rapidly rising nationally, reaching 88,000 new infections per day on average as the country enters its fifth wave, according to the latest data. 

Misgivings

The Centers for Disease Control and Prevention will hold a meeting later Friday to discuss clinical recommendations for who should now seek a vaccine.

The meeting should provide greater insight into the thinking of top federal advisors, some of whom have expressed misgivings about a "booster bonanza."

The vast majority of those people becoming hospitalized or dying with Covid are unvaccinated, and thus the best way to control the winter wave would be to reach those people, rather than topping up the vaccinated, the critics say.

A potential downside, they argue, is that vaccine holdouts might conclude the shots are ineffective.  

Another risk is a greater number of cases of vaccine-linked heart inflammation (myocarditis), especially among younger males. 

Both companies are conducting post authorization studies to assess the risks of myocarditis.

Experts are in broad agreement that boosters alone can't resolve the pandemic while the poorest countries, especially in Africa, remain stuck in the single digit percentages for people covered by their primary vaccination series. 

Last week, WHO chief Tedros Adhanom Ghebreyesus decried the fact that rich countries are administering six times more vaccine doses everyday than low-income countries are delivering primary doses.

This increases the risk of new variants of concern emerging in those regions, which could eventually evade the protective action of current vaccines.