Originally Published: 19 FEB 22 18:42 ET

Updated: 21 FEB 22 12:40 ET

By Jacqueline Howard, CNN

(CNN) -- As the world approaches the second anniversary of the declaration of the Covid-19 pandemic by the World Health Organization, on March 11, more nations are rolling out -- or are discussing the possibility of -- fourth doses of coronavirus vaccine for their most vulnerable. In the United States, leading public health officials say they are "very carefully" monitoring if or when fourth doses might be needed.

Israel was the first nation to roll out fourth doses, announcing in December that adults 60 and older, medical workers and people with suppressed immune systems were eligible to receive the extra shot if at least four months have passed since their third dose.

More recently, the Public Health Agency of Sweden announced last week that second booster doses are recommended for everyone 80 and older in the country. The United Kingdom's Department of Health and Social Care announced Monday that an extra booster dose of coronavirus vaccine will be offered in the spring to adults 75 and older, residents in care homes for older adults and immunosuppressed people 12 and older.

In the United States, health officials emphasized late last year that fourth doses were not yet needed and said it was too premature to be discussing a potential fourth dose of coronavirus vaccine for most people.

Now, the US Food and Drug Administration "is indeed continually looking at the emerging data on the pandemic and variants in the United States and overseas in order to evaluate the potential utility and composition of booster doses," FDA spokesperson Alison Hunt wrote in an email to CNN on Friday.

She confirmed that although Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, has noted that there is still much uncertainty as to how the pandemic may further evolve, he also has said it is possible that a fourth dose might be recommended as we move into fall.

A fall timeline coincides with the administration of flu shots, which could be convenient for people and makes sense scientifically because respiratory viruses -- like the coronavirus and influenza -- tend to peak in the winter months that follow.

"As more data become available about the safety and effectiveness of COVID-19 vaccines, including the use of a booster dose, we will continue to evaluate the rapidly changing science and keep the public informed," Hunt wrote. "Any determination that additional booster doses are needed will be based on data available to the agency."

If or when the FDA authorizes a fourth dose for the public, the next step would be for the US Centers for Disease Control and Prevention to review the data before making a recommendation for use, as the agency has done for other coronavirus vaccine recommendations.

'Vaccination and boosting will be critical'

The United States has seen significant improvements recently in Covid-19 cases, hospitalizations and deaths. As of Friday, cases were down 44% from the prior week, hospitalizations dropped 26%, and deaths were 13% lower, according to Johns Hopkins University data.

"Vaccination and boosting will be critical in maintaining that downward trajectory, particularly when you're talking about the red curve of severe disease leading to hospitalization," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said during a White House Covid-19 Task Force briefing Wednesday.

The "potential future requirement" for an additional boost or a fourth shot of the Pfizer/BioNTech or Moderna mRNA vaccines or a third dose of the Johnson & Johnson vaccine "is being very carefully monitored in real time," Fauci said. "And recommendations, if needed, will be updated according to the data as it evolves."

The CDC has no recommendation of fourth doses of coronavirus vaccine for the general public, but the agency updated its guidelines in October to note that certain people who are moderately or severely immunocompromised may receive a fourth dose of the Moderna or Pfizer/BioNTech vaccines.

"For the immunocompetent people, a single booster shot continues to provide high levels of protection against severe disease caused by Omicron," Fauci said Wednesday. "This should not be confused with the fact that for many immunocompromised people, already a second booster shot -- namely a fourth dose of an mRNA -- is recommended because of what we know about their poor response to the initial regimen."

Meanwhile, vaccine makers continue to study fourth doses in broader populations.

'We recognize the need to be prepared'

Pfizer announced in January that it has been studying the safety and efficacy of a fourth dose as part of its ongoing study of an Omicron-specific coronavirus vaccine among healthy adults ages 18 to 55.

For that study, participants have been separated into three cohorts. One includes 600 people who got three doses of the current Pfizer/BioNTech coronavirus vaccine before enrolling in the study. As part of the research, they will receive either a fourth dose of the current vaccine or a dose of the Omicron-based vaccine.

"While current research and real-world data show that boosters continue to provide a high level of protection against severe disease and hospitalization with Omicron, we recognize the need to be prepared in the event this protection wanes over time and to potentially help address Omicron and new variants in the future," Kathrin Jansen, senior vice president and head of vaccine research and development at Pfizer, said in the company's announcement of the study.

A study published by the CDC last week showed that protection against Covid-19 hospitalization and emergency department or urgent care visits is much higher after a third vaccine dose than a second dose but that protection wanes with time.

With the Omicron variant dominant in the United States, vaccine effectiveness was 87% against Covid-19 emergency department or urgent care visits and 91% against hospitalizations in the two months after a third dose. Effectiveness fell to 66% and 78%, respectively, by the fourth month, the data showed.

"Nonetheless, the level of 78 is still a good protective area," Fauci said Wednesday.

Dr. Christopher Murray, director of the University of Washington's Institute for Health Metrics and Evaluation, told CNN on Friday that he thinks having a fourth dose "makes very little sense" right now -- but possibly makes more sense this upcoming fall or winter.

"Because Omicron is going away," he said, "third dose immunity wanes, and so fourth dose will probably wane as well. So we would want to time a fourth-dose push for when either there's a new variant or for next winter. So I think right now, unless you're immunocompromised or you're older, unless you have some personal risk, it doesn't make a lot of sense."

The research showing some waning immunity after a third dose has led to more discussion around if or when fourth doses could be needed.

"Just like with everything else, the health departments look at this data that's coming out and wait for guidance from the CDC and from the drug manufacturers. The drug manufacturers have been monitoring vaccine effectiveness," Lori Tremmel Freeman, chief executive officer of the National Association of County and City Health Officials, told CNN on Friday.

The timing for fourth doses not only depends on waning immunity "but also depends on whether we see any more variants emerge and what we discover in terms of additional vaccine effectiveness for any emerging new infections," Freeman said.

If fourth doses are needed, "health departments would go into the mode of preparing to administer the fourth dose in the way they have done previous doses," Freeman said. However, of greater concern among local public health officers is the slow -- and declining -- pace of people completing their third doses, she said.

About 65% of the US population is fully vaccinated with at least their initial series, and about 28% of the population has received a booster dose, according to the CDC, as of Friday. But the pace of booster doses being administered has dropped to one of the slowest rates yet.

"As time goes on, if there is the necessity of a fourth dose, we're already behind with people getting the third dose," Freeman said. "So all of a sudden, we could have a fairly large segment of the population that is not up to date on vaccines because they're behind by two doses, potentially, and more people could get sick."