U.K. Prime Minister Boris Johnson said the variant of the coronavirus, B.1.1.7, which originated in the southeast region of the country late last year, could be more deadly than previously thought.

“In addition to spreading more quickly, it also now appears that there is some evidence that the new variant … may be associated with a higher degree of mortality,” Johnson said Friday afternoon during a press briefing.

Chief scientific adviser, Sir Patrick Wallace, added that the variant transmits 30% to 70% more easily, but there is no understanding of the reason yet, and there isn’t strong enough data to confirm the variant is, in fact, more deadly.

Wallace said the news about the virus being deadlier was based on looking at overall data, compared to just hospitalized patients. Hospitalized patients are not dying at increased rates with the new variant versus the old.

But when looking at data of positive cases, he said, “There is evidence that there is an increased risk for those who have the new variant compared to the old virus. Now, that evidence is not yet strong ... These data are currently uncertain and we don’t have a very good estimate of the precise nature, or indeed, if it has overall increased.”

The news shifts concerns about the variant, which had until now only been known to be highly transmissive. The strain accounts for just 1% of the cases in the U.S. to-date but has the potential to be a dominant strain in coming months.

At a White House briefing Thursday, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said vaccines are likely to still be effective against B.1.1.7 and the variant from South Africa, 501Y.V2, which has not yet been reported in the U.S.

“What we likely will be seeing is the diminution” in efficacy, more for the South African than the U.K. strain, but, “that does not mean vaccines will not be effective,” Fauci said Thursday.

Vaccine dosing flexbility

Meanwhile, the U.S. Centers for Disease Control and Prevention (CDC) has updated its guidelines on vaccine administration, allowing for delays for second doses and possible mixing of Pfizer (PFE)/BioNTech (BNTX) and Moderna (MRNA).

The idea was debated during the vaccine advisory committee meetings, with experts noting that more studies would need to be conducted to understand if the mixing and matching of doses would result in similar efficacy as two of the same doses. But as the vaccine rollout continues to be bumpy, the need to address potential short-term solutions — in lieu of more vaccine supply— has become necessary.