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Amid calls by both Pfizer and Moderna for a round of booster shots for those fully vaccinated against COVID-19, the U.S. Food and Drug Administration is signaling a reluctance to call for a third shot of a novel coronavirus vaccine.

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On Wednesday, Pfizer released briefing documents suggesting a third dose is needed for better protection against the virus, in advance of an FDA meeting on Friday.

The agency is set to consider and vote on Pfizer-BioNTech’s application for authorization of a third shot of the vaccine to people over age 16.

The agency’s decision will be sent to the Centers for Disease Control and Prevention panel for consideration.

In Pfizer’s documents, the company said the effectiveness of its vaccine is declining and a third dose is needed for maximum protection. According to its research, booster shots have been shown to return the Pfizer/BioNTech vaccine back to the 95% efficacy level seen in clinical trials.

“The high neutralizing titers against the Beta and Delta variants after a third dose … are encouraging,” Pfizer’s documents read. “These data suggest that a third dose could re-establish the efficacy levels shown 1 month after two doses, prolong duration of protection, and further increase the breadth of protection.”

To bolster its argument for a third dose of its vaccine, Pfizer also released results from a study conducted by Kaiser Permanente that showed the waning efficacy is likely due to the vaccine becoming less effective over time, and not because the new delta variant is more resistant to the vaccine.

Moderna also released on Wednesday data which suggested its COVID-19 vaccine is effective in preventing serious health issues or death from “variants of concern,” but showing that the vaccine’s efficacy decreases over time.

According to the results, those who received the vaccine 13 months ago were twice as likely as those who had received the vaccine eight months ago to have a breakthrough COVID-19 case — indicating the vaccine wanes over time.

A breakthrough case is one where a person who is fully vaccinated still contracts the virus.

Moderna noted that in its Phase 3 study conducted over the summer, 88 breakthrough cases of COVID-19 occurred in the more recently vaccinated group, compared to 162 cases in the group vaccinated last year.

Of the 250 breakthrough cases, only 19 were reported as severe cases.

The FDA also released ­­briefing documents Wednesday in advance of its meeting to consider Pfizer’s request for authorization of a third dose of the vaccine.

The agency said in its documentation that it did not see the convincing protection that a third dose might provide but went on to say it had not yet “independently reviewed or verified the underlying data or their (Pfizer’s) conclusions.”

“It should be recognized that while observational studies can enable understanding of real-world effectiveness, there are known and unknown biases that can affect their reliability. Due to these biases some studies may be more reliable than others,” the FDA’s documentation read.

A review released this week by an international group of scientists said that at this time, there is not enough evidence from studies “to show a need for boosting in the general population, in which efficacy against severe disease remains high.”

Two of the doctors in the group work for the FDA, but are leaving because they disagree with how booster shots are being recommended, according to The New York Times.