U.S. Department of Health and Human Services ADM Brett P. Giroir testifies before the House Committee on Energy and Commerce on the Trump Administration’s Response to the COVID-19 Pandemic, on Capitol Hill in Washington, DC, June 23, 2020.Kevin Dietsch | Pool via ReutersA senior administration official said Monday that U.S. health officials are “extremely encouraged” by the announcement that Pfizer’s coronavirus vaccine is highly effective in preventing Covid-19.The news, combined with progress in the nation’s manufacturing and distribution plans for Covid-19 vaccines, makes U.S. health officials “very optimistic” that the vaccine and others in development “could effectively end the pandemic,” Adm. Brett Giroir, an assistant secretary at the Department of Health and Human Services, told reporters on a conference call.”But until that vaccine is widely administered and distributed, we have much work to do,” he added. “I think we all realize our nation is in a critical phase in this pandemic, with significant community spread, cases averaging 100,000 per day over the last seven days and climbing, increase in hospitalizations with our three-day average estimated to be 61,000.”Earlier in the day, Pfizer announced its vaccine was more than 90% effective in preventing Covid-19 among those without prior infection, hailing the development as “a great day for science and humanity.” Comparatively, the Centers for Disease Control and Prevention says a vaccine for influenza reduces the risk of flu illness by 40% to 60% among the overall population.Pfizer CEO Dr. Albert Bourla told CNBC that the company is on track to produce up to 50 million vaccine doses in 2020, and up to 1.3 billion doses in 2021.”I believe this is likely the most significant medical advance in the last 100 years,” he said. “It is a great day for science. It is a great day for humanity when you realize your vaccine has 90% effectiveness. That’s overwhelming.”The announcement comes as the coronavirus continues to rapidly spread across the U.S. Over the past seven days, the country has reported an average of about 108,736 new cases per day, up more than 33% compared with a week ago, according to a CNBC analysis of Johns Hopkins University data. At least 30 states hit record highs in average daily new cases on Sunday, Hopkins data shows.Health officials fear the situation could become especially dire as people also get the seasonal flu and hospitals fill up. A coronavirus model from the Institute for Health Metrics and Evaluation, once cited by the White House, now projects more than 399,163 Covid-19 deaths in the U.S. by Feb. 1.A safe and effective vaccine is seen by investors and policymakers as a solution to get the global economy back on track after the pandemic wreaked havoc on nearly every country across the globe and upended businesses. There are currently four U.S.-backed frontrunners in phase three clinical trials: Pfizer, Moderna, AstraZeneca and Johnson & Johnson’s.Even with a vaccine, some infectious disease experts warn the coronavirus is likely to become as “endemic” as the annual flu virus.Giroir on Monday acknowledged that the coronavirus will likely never disappear from the planet. But he said he hopes that vaccines will bring the virus down to low levels and infections will be a rare occurrence, like polio.”Until that time, we have got to do the public health measures,” he said, urging people to wear masks, wash their hands and maintain their distance from others.