Here's promising news in the critical fight against Covid-19: The first formal review of a U.S. company's vaccine tests on people show that its injected medicine is more than 90 percent effective in preventing the disease. If the results hold up, that level of protection would put it on par with highly effective childhood vaccines for diseases such as measles. No serious safety concerns have been observed, says the company, Pfizer. It plans to ask the federal Food and Drug Administration soon for emergency authorization of the two-dose vaccine.

Pfizer expects to make enough doses by late December to immunize 15 million to 20 million people. "I think that likely, based on impact, this will be the greatest medical advance in the last 100 years," says Albert Bourla, the company’s chief executive. Dr. Anthony Fauci, a prominent presidential adviser on the pandemic, estimates that the vaccine will be available to all Americans "probably by April." He calls the breakthrough "extraordinarily good news."

Pfizer used gene technology to develop a vaccine that uses the body's cells to make proteins that trick the immune system to react as if invaded by the Covid virus, training it for the real thing. Results of the worldwide trial using 43,538 volunteers since late July means the disease could be suppressed eventually, though it's unclear how long the vaccine will protect people against the virus. "The end of the battle against Covid-19 is still months away," says President-Elect Joe Biden. In the meantime, Covid cases are climbing steadily in all states – closing in-person education in some areas and bringing new warning to wear masks, avoid crowds and celebrate Thanksgiving only with small family meals. This country has recently seen more than 100,000 new Covid-19 cases daily.

President-elect says: "I congratulate the brilliant women and men who helped produce this breakthrough." – Joe Biden

Doctor says: "This development doesn’t mean we can all relax and start doing more things. It means we need to tighten up even further until the vaccine becomes available." – Dr. Aaron Carroll, professor of pediatrics, Indiana University School of Medicine

Expert says: "The key question still centers upon time. Will time tell us that the protection remains useful?" – Michael Kinch, a drug development expert at Washington University in St. Louis