Only 36% of American seniors and just 16% of the population aged 18 and over received a booster dose of the vaccine through the program, which became available in September.

According to doctors, the effectiveness of vaccines decreases over time. Even vaccinated people who have not been vaccinated for some time may not be sufficiently protected against serious infections.

All the evidence that has emerged over the past two years suggests that vaccines are effective. But the effectiveness has changed as the virus has evolved. It also disappears if the patient has not had the last vaccination for some time. This is especially true for elderly vulnerable people.

Studies have shown higher efficacy of booster shots among adults aged 65 years and older. Compared to unvaccinated people, the updated vaccine was 84% effective against hospitalizations.

Vaccines are much less effective in preventing common infections. Some scientists say the value of boosters may be more limited in younger people, who are generally not at risk for severe cases.