The World Health Organization has said it was not clear whether Covid-19 booster vaccines will be needed to maintain protection, until further data is collected.

It comes as Pfizer and BioNTech announced they will seek regulatory approval for a third dose of their Covid-19 vaccine.

"We don't know whether booster vaccines will be needed to maintain protection against Covid-19 until additional data is collected, but the question is under consideration by researchers," the WHO said in a reply to a Reuters query.

"There is limited data available on how long protection from current Covid-19 vaccine doses lasts and whether an additional booster dose would be beneficial and for whom," it said.

The pharmaceutical companies say that, based on drop-offs in efficacy seen in Israel after six months, they believe that a third dose may be needed within six to 12 months after full vaccination.

"While protection against severe disease remained high across the full six months, a decline in efficacy against symptomatic disease over time and the continued emergence of variants are expected," a statement from the two companies said.

It comes after initial data from a trial showed a third shot pushed antibody levels five to ten times higher against the original strain and the Beta variant, compared to the first two doses alone, according to the statement.

"The companies expect to publish more definitive data soon as well as in a peer-reviewed journal and plan to submit the data to the FDA (Food and Drug Administration), the EMA (European Medicines Agency) and other regulatory authorities in the coming weeks," the statement said.

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The companies expect that a third dose will perform similarly well against the highly transmissible Delta strain, which is quickly becoming globally dominant.

Out of caution, they are also developing a Delta-specific vaccine, the first batch of which has been manufactured at BioNTech's facility in Mainz, Germany.

The companies anticipate the clinical studies will begin in August, subject to regulatory approvals.