The Biden administration has been planning Covid-19 booster shots for weeks, as the delta variant drives a new pandemic wave across the country. The debate erupted last month when top officials from the Food and Drug Administration resigned over reported concerns that the decision-making process was being driven by politics rather than science.

Based on data indicating that vaccine effectiveness wanes over time, Israel has already pushed ahead with booster shots, with the majority of the country’s population currently eligible for a third shot. The country is even bracing for the possibility that patients will require a fourth injection. The data coming out of Israel has reportedly influenced the Biden administration, and it appeared for a time that the US would quickly follow suit. President Joe Biden proposed in a speech last month that every vaccinated American receive a booster shot, subject to FDA approval.

This argument has guided actions taken in recent days by the FDA and the Centers for Disease Control and Prevention. Following the advice of its vaccine advisory board, the FDA approved a third dose of Pfizer’s vaccine for people over 65 and those at higher risk due to their work or medical conditions. A CDC committee also recommended that booster shots be targeted primarily at people over the age of 65. The committee did not specify prioritizing people based on their job after some debate, but the CDC’s director overruled the panel and said workers at high occupational risk should be included.

That assessment of the evidence was shared by other experts I spoke with. Although there may be some waning effectiveness against any symptomatic disease, vaccines continue to do an excellent job of preventing hospitalizations and deaths for the vast majority of people.

Many experts believe boosters are appropriate for at least some people, and federal officials appear to agree.

The question has been whether they should be prioritized for all vaccinated people, as Biden announced last month, or limited to certain vulnerable populations. Should the United States concentrate its vaccine supply on boosting all of its citizens, or should it do more to share vaccines with the developing world, where vaccination rates remain significantly lower than in the United States and Europe? That is to be expected given that three out of every four adults in the United States have received at least one dose of the Covid-19 vaccine. There are simply more vaccinated people than unvaccinated people, and while vaccines provide excellent protection against infection, they are not without flaws. The virus does get through on occasion, but vaccinated people are much less likely to be hospitalized or die from it.

A recent CDC study tracked new Covid-19 cases and hospitalizations in New York state from early May to late July. The study period includes the transition from the “alpha” variant to the “delta” variant, which became dominant by the beginning of July, but only a portion of the recent surge in reported cases.

The CDC researchers concluded that as the delta variant took over, the Covid-19 vaccines became slightly less effective in preventing any illness. Vaccines were estimated to be 90 percent effective at preventing new infections in May. However, by mid-July, the estimated effectiveness had fallen to just under 80%. Vaccinated people were more likely to become infected and become ill at that point.

Another CDC study, which ran through July, looked at national data to see if the Pfizer/BioNTech and Moderna vaccines were becoming less effective at preventing severe illness over time. It discovered, like the New York study, that vaccines are extremely effective at their most important job — preventing hospitalization due to Covid-19. They found no significant decrease nearly six months after patients received a second dose of the vaccine. A newer CDC study discovered some differences in the effectiveness of Moderna and Pfizer in preventing severe illness over time, with the former outperforming the latter, but overall effectiveness for both vaccines remained high.

People who received the Johnson & Johnson vaccine, which was shown to be less effective at preventing symptomatic illness, may also benefit from boosters, though long-term data on that vaccine’s effectiveness is still being collected. The vaccine was found to be 71% effective in preventing hospitalizations from March to August 2021, significantly lower than Moderna or Pfizer.