Doctors are excited but cautious about Pfizer's COVID-19 vaccine

Even with early promising results from Pfizer's Covid-19 vaccine trial, significant challenges and unanswered questions remain before average Americans can get a shot. Pfizer's vaccine is a new type of technology that's never been used in mass human vaccination before and experts caution that much remains unknown about its safety, how long it might work and who might benefit most.

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Pfizer's phase 3 clinical trial began in late July and has enrolled more than 43,000 study participants. Some received the actual vaccine, while other received a placebo. Vaccine trials rely on a certain number of infections to occur. If more infections are reported among study participants who received the placebo than the actual vaccine, it's a good signal of efficacy.

On Monday, the U.S. pharmaceutical giant Pfizer and its German partner, BioNTech SE, said that early analysis show that the Covid-19 vaccine is more than 90 percent effective at preventing symptomatic illness.

The finding exceeded expectations and was met with enthusiastic optimism — along with many caveats.

That's because the drugmaker has not released full details on its preliminary analysis.

Pfizer stated that about a third of its study participants have "racially and ethnically diverse backgrounds" but didn't reveal if there was stronger protection for particular age groups.

"We don't know anything about groups they didn't study, like children, pregnant women, highly immunocompromised people and the eldest of the elderly," Dr. Gregory Poland, director of the Mayo Clinic's Vaccine Research Group in Rochester, Minnesota, said. And would people previously sick with Covid-19 be protected against reinfection? That remains unclear.

What's more, the Pfizer vaccine uses a brand new technology called messenger-RNA, or mRNA. It has never been approved for human vaccination before. Instead of using bits of virus to provoke an immune response, the mRNA trains the immune system to target the spike protein found on the surface of the coronavirus.

The spike is what allows the virus to invade human cells. In theory, blocking the spike would mean people wouldn't become infected with the virus.

"The exciting thing is that these types of vaccines can be manufactured pretty quickly," said Dr. Richard Besser, president of the Robert Wood Johnson Foundation and former acting director of the Centers for Disease Control and Prevention.

"The caution is that this would be the first mRNA vaccine to be used in humans, so it's important to look carefully at the safety data."

It is also important to get a sharper picture of Pfizer's claim of about 90 percent effectiveness. This first analysis only included data on 94 confirmed Covid-19 cases, meaning there is no proof yet that the vaccine prevented infection.

Those who had the actual vaccine, rather than the placebo, were overwhelmingly likely to have few to no symptoms. That is, the vaccine seems to have prevented major Covid-19 complications.

It is unclear whether people who received the vaccine were less likely to be contagious.

Still, the unanswered questions should not squash early excitement about the findings, which seem to indicate that the vaccine may have the ability to transform a serious illness into something that resembles the common cold.

"The infections that we're really focusing on are the ones that lead to hospitalization," said Dr. Robert Citronberg, executive medical director of infectious disease and prevention at Advocate Aurora Health in Illinois. "If those are the ones this vaccine can target, that's a huge win."

Dr. William Haseltine, an infectious diseases expert and president of ACCESS Health International, agreed with that assessment. He predicted that if the vaccine indeed is proven safe and effective, it may work more like the flu shot.

"It's not necessarily going to protect you from infection, and it may not work for everyone," Haseltine said Monday on MSNBC. "But it should be useful for many people. And it should moderate the severity of disease."

It's also uncertain how long such protection might last. That answer can only come with time, as it's impossible to know yet whether immunity remains for months, a year, two years — or a lifetime. (For comparison, flu shots are generally only about 50 percent effective, while measles vaccines are up to 97 percent effective.)

Pfizer's chief executive officer, Albert Bourla, told CNBC Monday that the drugmaker will follow participants for two years to analyze safety and ongoing protection.

"As time progresses, we will find out about the durability of the protection," Bourla said. "We will see how long the immunogenicity lasts and how long the cell immunity lasts."

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Pfizer has said it will not apply for emergency use authorization of its vaccine candidate until it has collected two months of safety information following the final dose of the vaccine. Pfizer's vaccine requires two doses, about a month apart.

That's likely to occur next week, when it is expected to have that safety data. There have been no serious safety concerns so far.

Alex Azar, head of the Department of Health and Human Services, told Fox News Channel Monday that the FDA will conduct an "independent review process."

Besser, formerly of the CDC, urged caution until "independent eyes" have had a chance to scrutinize the details of the findings. "This is just one data point in a number of data points," he said.