Novavax — which has never produced a vaccine before — declined to answer specific questions about its product’s purity levels and whether it had been successful in addressing its longstanding manufacturing issues.

Unlike Pfizer and Moderna, the first manufacturers to launch vaccines using rapidly produced messenger RNA, Novavax is employing the previously used but complicated approach of creating the key ingredient with bug cells. While the process, which involves infecting the cells to produce spike proteins, is familiar to scientists, it is difficult to scale.

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U.S. officials working with the company are not as confident, according to three people with knowledge of the matter. Novavax’s manufacturing problems are seen as far more difficult to fix than the sanitary and design concerns that halted production of J&J’s vaccine at the Emergent plant earlier this year, those people said.

And even as the company begins to seek regulatory approval in other countries, there remains doubt in the U.S. that it has solved the fundamental vaccine purity flaws that the people with knowledge said have affected its ability to make doses at plants around the world.

Several vaccine batches have already been discarded, and four people with knowledge of the matter say U.S. officials now no longer expect the company to win FDA sign-off on the vaccine until next year at the earliest.

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