On Monday, Pfizer marked a milestone in society’s fight against the coronavirus pandemic, announcing its experimental vaccine was highly effective at preventing COVID-19 in a 43,538-person study.

The $220 billion drugmaker said a two-dose regimen of its shot was found to be more than 90% effective in preventing COVID-19, based on 94 cases of the disease observed in the large-scale trial.

But even with the good news, there are caveats and unanswered questions for Pfizer and its German partner BioNTech.

William Haseltine, a longtime biotech executive and infectious-disease expert, told Business Insider he wanted to see the underlying data to support the efficacy claim.

Haseltine has previously criticized other front-runners in the race for a coronavirus vaccine, namely Moderna, for touting study results in news releases before releasing detailed data. With Pfizer’s release on Monday, however, Haseltine said there wasn’t any data in the release.

The analysis was based on 94 cases of COVID-19 among study participants, but Pfizer didn’t share an exact breakdown of how many got sick from getting Pfizer’s vaccine versus the placebo. The release also didn’t specify how many of the cases were severe or mild or if different age groups had varying levels of protection.

Beyond saying there have been no serious safety concerns, Pfizer didn’t provide any details on the safety profile, such as the frequency and severity of typical side effects. CEO Albert Bourla said in a statement the company would share additional efficacy and safety data “in the coming weeks.”

“This is science by public pronouncement,” Haseltine said.

Haseltine is a former Harvard medical professor who founded two research centers focused on HIV/AIDS and cancer at the school. The virology and infectious-disease expert is now the chairman and president of Access Health International, a nonprofit healthcare think tank. He has also founded and led several biotech companies, including Human Genome Sciences, which was eventually bought by GlaxoSmithKline for $3 billion.

During the pandemic, he has advocated the US to embrace an expansive testing strategy, previously saying the country’s response was overly reliant on a vaccine.

“It is very welcome news that the vaccine has a measurable effect,” Haseltine said, adding there was still much to learn about Pfizer’s shot.

“There are many, many outstanding questions which are left unanswered,” he added.

Most of the questions have to do with the limitations of the study. The trial was designed to see if there were fewer cases of symptomatic COVID-19, the disease caused by the coronavirus, in people getting the vaccine rather than placebo.

It brings up one crucial distinction that could have a major influence on the pandemic response: Does this vaccine prevent infection as well as disease? 

Pfizer’s trial, and the ongoing studies of other leading coronavirus-vaccine developers, aren’t regularly testing volunteers to gauge asymptomatic infections. That may mean vaccinated people could still become asymptomatic carriers and unknowingly spread the virus to others. 

“That’s a major point that I don’t think most people appreciate,” Haseltine said. “It doesn’t mean an end to the epidemic.”

Haseltine also raised the question of if the vaccine reduces serious disease and ultimately affects the number of hospitalizations and deaths.

Again, the study’s findings are limited by its main goal, which did not distinguish between a mildly ill COVID-19 patient — maybe someone with a minor fever and cough for a few days — and someone who is critically ill.

Finally, Pfizer’s news release made no mention of if the vaccine appeared as effective in different subgroups, such as older people, who are more susceptible to the worst outcomes of the virus.