A new CDC presentation said the data needed to properly evaluate Covid-19 vaccine booster shots for the general population is limited – even as U.S. President Joe Biden pressures health officials who do To release vaccinations for wide distribution from the week of September 20th.

Slides released prior to a presentation at a meeting of the Centers for Disease Control and Prevention Advisory Group on Monday suggest the panel may limit its initial approval of additional shots to vulnerable groups and healthcare workers.

A nurse vaccinates 15-year-old Sherri Trimble at a vaccination clinic at Health First Medical Center in Melbourne, Florida.

Paul Hennessy | SOPA pictures | LightRakete | Getty Images

According to the CDC scientist Dr. Slides submitted to Sara Oliver suggest several studies suggest that the approved Covid vaccines may still be effective in preventing serious illness and hospitalization, but may be less effective in preventing infection or mild symptomatic illness. The CDC’s Advisory Committee on Vaccination Practices will meet Monday to consider booster vaccinations for all eligible Americans. They are also due to vote on final approval of the Pfizer vaccine by the Food and Drug Administration.

Since the highly contagious Delta variant first appeared, the vaccine’s effectiveness ranged from 39% to 84%, according to the presentation, which referred to several separate studies. A study looking at health workers and first responders showed the vaccine’s overall effectiveness dropped to about 65% in July – from about 90% in February, according to the slides. Israeli data shows that the effectiveness of Pfizer’s vaccine in this country has dropped as low as 39%.

Scientists have said that the vaccines become less effective over time, and the Delta variant is a more resilient strain that is able to break through that protection.

It is “important to monitor efficacy trends based on disease severity over time,” the slides read.

It was pointed out on the slides that vaccines often require multiple doses. Hepatitis B and HPV vaccinations, for example, require a third dose after six months.

“Vaccines that require more than one dose don’t necessarily mean that an annual booster is required,” Oliver said during the presentation.

Once booster shots are available, nursing home residents, health care providers, and the elderly – the first groups to be vaccinated in December and January – will likely be given priority for the additional vaccinations, according to the CDC slides.

The CDC stressed that vaccinating the unvaccinated should be a “top priority” and giving booster doses to vaccinated individuals should not deter those who remain unprotected from the virus.

The agency also emphasized the importance of vaccine availability around the world.

“Uncontrolled worldwide spread that could lead to new variants threaten the control of the pandemic everywhere,” said Oliver. In addition to global distribution, policy on boosters “should also consider equity in the US population,” she added.

The meeting on Monday comes after President Joe Biden said Friday that U.S. regulators are considering giving Covid booster vaccinations five months after the basic vaccinations are completed and are bringing forward the expected schedule for a third vaccination by three months.

Scientists have sharply criticized the Biden administration’s push to distribute booster syringes widely, saying that the data provided by federal health officials is not convincing enough to currently recommend a third vaccination to most of the American population.

The Biden administration has publicly stated that the third dose will not be carried out without FDA approval and a vote by ACIP.