The battle for Victory over Covid-19 has received a major boost as Pfizer and partner BioNTech said Monday that their vaccine against Covid-19  was strongly effective, exceeding expectations with results that are likely to be met with cautious excitement — and relief — in the face of the global pandemic.

The vaccine is the first to be tested in the United States to generate late-stage data. The companies said an early analysis of the results showed that individuals who received two injections of the vaccine three weeks apart experienced more than 90% fewer cases of symptomatic Covid-19 than those who received a placebo. For months, researchers have cautioned that a vaccine that might only be 60% or 70% effective.

The Phase 3 study is ongoing and additional data could affect results.

In keeping with guidance from the Food and Drug Administration, the companies will not file for an emergency use authorization to distribute the vaccine until they reach another milestone: when half of the patients in their study have been observed for any safety issues for at least two months following their second dose. Pfizer expects to cross that threshold in the third week of November.

“I’ve been in vaccine development for 35 years,” William Gruber, Pfizer’s senior vice president of vaccine clinical research and development, told STAT. “I’ve seen some really good things. This is extraordinary.” He later added: “This really bodes well for us being able to get a handle on the epidemic and get us out of this situation.”

Although it is a bright spot in the battle against the pandemic and a triumph for Pfizer and BioNTech, a German company, key information about the vaccine is not yet available. There is no information yet on whether the vaccine prevents severe cases, the type that can cause hospitalization and death.

Nor is there any information yet on whether it prevents people from carrying the virus that causes Covid-19, SARS-CoV-2, without symptoms.

Without more information, it’s too early to start predicting how much of an impact the vaccine could make, said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy.

“I don’t want to dampen any enthusiasm for this vaccine. I just want us to be realistic,” Osterholm said. “For a vaccine to really have maximal impact, it’s going to have to also reduce severe illness and death. And we just don’t know yet.”

Because the vaccine has been studied for only a matter of months, it is impossible to say how long it will protect against infection with the virus. The vaccine does cause side effects, including aches and fevers, according to previously published data. Gruber said that he believed the side effect profile was comparable to standard adult vaccines, but probably worse than Pfizer’s pneumonia vaccine, Prevnar, or a flu shot.

The results have not been peer-reviewed by outside scientists or published in a medical journal, and even Pfizer and BioNTech have been given no other  details about how the vaccine performed by the independent monitors overseeing the study.

Initial supplies of the vaccine, if authorized, will be limited. Pfizer says up to 50 million doses could be available globally. by the end of the year, with 1.3 billion available in 2021. There are also expected to be distribution challenges. The vaccine must be stored at super-cold temperatures, which could make it extremely difficult to deliver to many places. Pfizer has said it is confident those issues can be managed.

Although the estimate of the efficacy of the vaccine could change as the study is completed, it is close to a best-case scenario. That also bodes well for other vaccines in the late stages of testing, including those developed by Moderna, AstraZeneca, and Johnson & Johnson.

Both Pfizer’s vaccine and Moderna’s use messenger-RNA, or mRNA, technology, which uses genetic material to cause the body to create a protein from the virus; the immune system then recognizes the virus and learns to attack. Other vaccines in the late stages of development use genetically engineered viruses for a similar purpose, or pieces of protein that are directly injected. No mRNA product has ever been approved by regulators.

The story of how the data have been analyzed seems to include no small amount of drama. Pfizer, seeing an opportunity to both help battle a pandemic and demonstrate its research prowess, made decisions that were always likely to make its study the first of a Covid-19 vaccine to produce data — including its decision to have an independent group of researchers, known as a data safety and monitoring board, take an early look at the data in the 44,000-volunteer study before its completion.

The first analysis was to occur after 32 volunteers — both those who received the vaccine and those on placebo — had contracted Covid-19. If fewer than six volunteers in the group who received the vaccine had developed Covid-19, the companies would make an announcement that the vaccine appeared to be effective. The study would continue until at least 164 cases of Covid-19 — individuals with at least one symptom and a positive test result — had been reported.

That study design, as well as those of other drug makers, came under fire from experts who worried that, even if it was statistically valid, these interim analyses would not provide enough data when a vaccine could be given to billions of people.

In their announcement of the results, Pfizer and BioNTech revealed a surprise. The companies said they had decided not to conduct the 32-case analysis “after a discussion with the FDA.” Instead, they planned to conduct the analysis after 62 cases. But by the time the plan had been formalized, there had been 94 cases of Covid-19 in the study. It’s not known how many were in the vaccine arm, but it would have to be nine or fewer.

Gruber said that Pfizer and BioNTech had decided in late October that they wanted to drop the 32-case interim analysis. At that time, the companies decided to stop having their lab confirm cases of Covid-19 in the study, instead leaving samples in storage. The FDA was aware of this decision. Discussions between the agency and the companies concluded, and testing began this past Wednesday. When the samples were tested, there were 94 cases of Covid in the trial. The DSMB met on Sunday.

This means that the statistical strength of the result is likely far stronger than was initially expected. It also means that if Pfizer had held to the original plan, the data would likely have been available in October, as its CEO, Albert Bourla, had initially predicted.

Gruber said that there will not be another interim analysis conducted in the study. He also said that Pfizer’s estimate that it could file for authorization of the vaccine by the third week of November was based on the assumption that the FDA would be willing to accept two-month safety data on half the volunteers in the study as initially planned, when it was to include 30,000 volunteers, not more than 44,000, as is now the case. Those discussions are ongoing.

But Gruber said he now expects that by the time of the planned meeting of the FDA’s vaccine advisory committee in December, the study’s efficacy portion could be completed, having reached 164 cases of Covid-19.

He also emphasized that although there will only be a few months of data from this study, results from earlier studies make him optimistic that immunity from the vaccine will not wane rapidly.

The study has enrolled  43,538 volunteers, the companies said, and 38,955 have received their second dose. About 42% of global participants and 30% of U.S. participants have racially and ethnically diverse backgrounds.

Bourla, Pfizer’s CEO, said the results mark “a great day for science and humanity,” in a statement, saying they provide “initial evidence of our vaccine’s ability to prevent Covid-19.” He added: “We look forward to sharing additional efficacy and safety data generated from thousands of participants in the coming weeks.”