The United States surpassed 10 million coronavirus cases on Monday, just 10 days after hitting 9 million. The average number of daily new infections has exceeded 100,000, and public health experts warn the country is entering the pandemic’s worst phase yet.

The United States hit the milestone as Pfizer announced its coronavirus vaccine candidate was more than 90 percent effective, compared with a placebo. Epidemiologists and health experts were optimistic about the results, but also cautioned that more information is needed on the vaccine’s long-term efficacy and safety.