The Centres for Disease Control and Prevention still has to make its recommendations and has said three shots still provide strong protection against severe disease and death.
FDA vaccine chief Dr Peter Marks said data showed that protection could wane and a fourth shot "will help save lives and prevent severe outcomes". The age of 50 was chosen as that's when chronic health issues become more frequent, adding to Covid's risks.
Epidemiologist Dr John Brownstein, of Boston Children's Hospital, told ABC News: "We will likely have another surge, whether sooner or later we don't quite know, but there is a recognition that, over time, immunity wanes. And booster protection is at least helpful for those older and with underlying conditions."
In New Zealand, health authorities have been considering the fourth booster with Pfizer. Take-up of the third booster remains sluggish despite the grim death toll here, partly because people who have been infected with Omicron have to wait for three months to get one.
There's a debate to be had over where the cut-off should be for a fourth booster. Perhaps they should be available to any adult who wants one.
In the US, Pfizer/BioNTech had sought the fourth dose for Americans aged 65 plus and Moderna wanted authorisation for people aged 18 and older.
Israel, which is providing the most data on fourth shots, began administering the extra dose to people with compromised immunity and those aged 60 and over, before widening it to all adults, four months after the last shot.
During the Delta and Omicron waves in New Zealand, people aged 70 and over have made up only 3 per cent of cases but the most, at 23 per cent, of those hospitalised with Covid.
People in their 50s and 60s have the next lowest case percentages - 9 and 5 - and are also more likely to have severe infections with a combined 24 per cent of hospitalisations.
Younger people aged 20 to 49 make up 53 per cent of confirmed cases and 40 per cent of hospital cases.
That would appear to suggest there's a case for an age 50-plus booster, at least. Most of New Zealand's deaths with or from Covid have occurred in the 70 plus age group. But there have been more than 50 linked deaths in the 50 to 70 age group.
Director general of health Dr Ashley Bloomfield has said that the aim at this stage, should the fourth dose be approved by the Government, would be to top-up protection for people most at-risk, and for those who were vaccinated first, rather than the wider population.
There's an argument to be made that others should be able to have it as well. The pandemic is still not over - another variant could quickly emerge and it would get here fast with open borders and it's better to be proactive and prepared than reactive in that scenario.
And with people wanting to do a bit more with their lives, a booster makes sense before travel if your last shot was four to six months ago.
Annual shots may be needed in coming years but living with Covid likely means a second booster this year.
