The United States is closely watching the more infectious variant of COVID-19 after British officials warned that it may also be more deadly, two top US health officials said on Saturday, cautioning more data is needed.

Officials are somewhat more worried about a separate variant from South Africa, although it has not yet been identified among US cases of the novel coronavirus, National Institutes of Health (NIH) Director Francis Collins and Dr Anthony Fauci, President Joe Biden’s top COVID-19 medical adviser, also said.

Collins noted the UK’s data was preliminary, and said it was unclear why those with the UK variant faced a higher risk of death, whether by changes in the virus itself or other external causes such as pressures on the healthcare system.

“Let’s take this as something to watch closely,” he told MSNBC in an interview.

Fauci separately told MSNBC that he needed to see the raw data from the UK before fully assessing the mortality risk and that US officials were weighing how the two new strains could impact vaccine efficacy.

Their comments come as Democratic President Joe Biden takes the reins of the nation’s COVID-19 response, seeking to re-energize the fight against the highly infectious disease and urging a war-like stance.

In a flurry of actions since taking office Wednesday, Biden unveiled a new US strategic plan to curtail the outbreak and signed numerous executive orders to boost vaccines and increase mask use, among other measures.

The United States is the nation hardest-hit by COVID-19, with 24.70 million cases and 413 775 deaths by midnight Friday. Over the past week, 3 089 Americans have died of the disease on average, while 20 US states reported record deaths this month.

British Prime Minister Boris Johnson on Friday warned that the UK variant was associated with higher level of mortality.