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Scientists at the Food and Drug Administration said the Covid-19 vaccine developed by the biotech Novavax was effective at preventing disease but questioned how effective it is against currently circulating strains — and raised concerns about the potential for a rare side effect.

Their analysis, laid out in briefing documents released Friday ahead of an advisory committee meeting on the vaccine, sent Novavax shares down more than 16%.


The FDA’s scientists raised concerns, in particular, about the risk of myocarditis and pericarditis associated with the vaccines. Both are types of inflammation of the heart or surrounding tissue that have also been associated with the mRNA-based Covid vaccines developed by Pfizer and its partner BioNTech and Moderna.

The FDA’s Vaccines and Related Biological Products Advisory Committee will meet on June 7 to consider whether the Novavax vaccine should be authorized as another Covid vaccine option in the U.S. The agency is not bound by the recommendations of its outside advisory committees, but tends to follow them.

There has been interest in the Novavax vaccine because it is produced on a more traditional vaccine platform than the newer mRNA vaccines, which might convince some people who have not received a Covid vaccine to get one. The Novavax vaccine is composed of a key protein from the SARS-CoV-2 virus that is produced in insect cells and bolstered with another chemical, called an adjuvant, meant to produce an immune response. Similar technologies are used in some influenza vaccines.


But the FDA documents raise questions about how the vaccine, known as NVX-CoV2373, will be positioned if it is authorized.

“Relevant data to assess effectiveness of NVX-CoV2373 against the Omicron variant and sublineages, including observational data from use in other countries where the vaccine has been deployed, are currently unavailable,” the FDA wrote. “However, based on the efficacy estimate in the clinical trial of this vaccine, it is more likely than not that the vaccine will provide some meaningful level of protection against COVID-19 due to Omicron, in particular against more severe disease.”

It is also not clear how much a booster dose of the vaccine, currently given as a two-dose series, would increase protection.

Five of the cases of myocarditis and pericarditis in the Novavax trial were reported within two weeks of vaccination. One case may have been caused by Covid, not the vaccine, but there were no clear alternative explanations for the other cases. Four cases of heart inflammation occurred in young men, mirroring what is seen with the mRNA vaccines.

What’s more, the FDA scientists said, the identification of multiple vaccine-associated cases in the safety database with the Novavax vaccine “raises concern that if causally associated, the risk of myocarditis following NVX-CoV2373 could be higher than reported during post-authorization use of mRNA COVID-19 vaccines.” The FDA scientists noted that myocarditis was not identified before the mRNA vaccines were authorized.

The FDA also said that an authorization of the Novavax vaccine would depend on information about the company’s manufacturing processes.

“Testing and submission of manufacturing and product information for the NVX-CoV2373 product intended for use under EUA were still in process at the time of this review,” the FDA scientists wrote.

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