U.S. President-elect Joe Biden has hailed as a “breakthrough” Pfizer’s announcement that its vaccine data suggests the shots may be 90% effective at preventing COVID-19.

Biden on Monday congratulated those involved in giving the country “such cause for hope.”

But at the same time, Biden noted that the end of the battle against COVID-19 is still months away.

He said even if a vaccine is approved by the end of this month and some Americans are vaccinated later this year, it’ll be many more months before there’s widespread vaccination across the country.

Biden cited a warning by the head of the U.S. Centers for Disease Control and Prevention that for the foreseeable future, a mask remains a more potent weapon against the virus than the vaccine.

“Today’s news doesn’t change this urgent reality,” Biden said, adding that Americans will have to rely on masking, distancing, social tracing, hand washing and other measures to keep themselves safe well into next year.

He said the U.S. is still losing over 1,000 people a day from COVID-19 and will continue to get worse unless progress is made on mask-wearing and other actions.