On Wednesday, U.S. health officials announced plans to give COVID-19 booster shots to all Americans in order to strengthen their immunity in the face of the rising delta variant and evidence that the vaccines’ effectiveness is dwindling.

The plan, as laid out by the director of the Centers for Disease Control and Prevention and other top officials, calls for an additional dose eight months after people receive their second dose of the Pfizer or Moderna vaccine. The doses could begin as early as the week of September 20.

According to health officials, people who received the single-dose Johnson & Johnson vaccine will most likely require additional shots. However, they stated that they are awaiting additional data and are still working out the details. According to officials, the overall plan is awaiting a Food and Drug Administration evaluation of the safety and effectiveness of a third dose.

The World Health Organization’s top scientists vehemently opposed the US plan, pointing out that poor countries are not receiving enough vaccine for their initial rounds of shots.

Dr. Soumya Swaminathan, the organization’s top scientist, stated, “We believe clearly that the data does not indicate that boosters are required”, for everybody. She warned that failing to vaccinate billions of people in the developing world could lead to the emergence of new variants and “even more dire situations.” WHO officials have repeatedly expressed concern that variants will continue to emerge in areas where the virus is not being controlled by measures such as vaccination. They have called for vaccine equity and international “solidarity.”

She stated that the United States is “by far” the largest contributor to the global fight against COVID-19 and will “continue to be the world’s arsenal for vaccines.” She also stated that the United States has enough vaccine to provide boosters to all Americans.

In recent weeks, public health experts in the United States have become concerned about the rapid spread of the highly contagious variant and reports of “breakthrough” cases in vaccinated people.

The CDC also released three studies conducted during the delta surge that suggest that the COVID-19 vaccines are still highly effective at keeping Americans out of the hospital, but their ability to prevent infection is declining significantly among nursing home patients and others.

However, the new studies, on their own, fall short of the kind of data that some experts believed would be required for such a recommendation.

Some researchers have been looking for signs that hospitalizations or deaths are on the rise as a necessary indicator that boosters may be required. The new studies, on the other hand, discovered no difference in vaccine effectiveness against hospitalizations.

The study that looked at reported infections in nearly 15,000 nursing homes and other long-term care facilities was the most directly related to a possible need for boosters of the three studies released by the CDC on Wednesday.

It discovered that the effectiveness of the Pfizer and Moderna vaccines against COVID-19 infections dropped from around 74% in March, April, and early May to 53% in June and July. According to the researchers, it is unclear how much of the decline is due to the delta variant and how much is due to a more general weakening of immunity that could have occurred against any strain.

All infections, with or without symptoms, were included in the study. More research is needed, according to the researchers, to determine if there was a higher incidence of infections that resulted in severe illness.

Another study looked at hospitalizations at 21 different hospitals. It discovered that vaccine effectiveness against COVID-19-associated hospitalization was 86 percent two to 12 weeks after the second dose, and 85 percent 13 to 24 weeks after.

The third study discovered that hospitalization protection remained constant at around 95% for nearly three months. In addition, the effectiveness of the vaccine against new laboratory-confirmed infections for all adults in New York state fell from about 92 percent in early May to about 80 percent in late July.

The researchers are unsure why the decline occurred, but it coincided with the delta variant as well as a relaxation of social distancing and mask rules.