Pfizer and their partner BioNTech indicated Monday that their COVID-19 vaccine is strongly effective and exceeded their expectations. Pfizer’s senior vice president of vaccine clinical research and development, William Gruber, states, “I’ve seen some really good things. This is extraordinary.” However, these exciting results are also met with caution. There’s a phase 3 study currently ongoing and additional data could result in different, less promising data regarding effectiveness, according to Stat News.

Pfizer’s vaccine is the first coronavirus vaccine to be tested in the United States to generate late-stage data. An early analysis of the data from the trial indicates individuals who received the vaccination twice three weeks apart displayed over 90 percent fewer cases of symptomatic COVID-19 infections than those that received a placebo. This comes after Pfizer cautioned that the vaccine would likely be only 60 to 70 percent effective at best.

There are still many holes in the data collected. There is no information regarding effectiveness in preventing severe cases and asymptomatic cases of COVID-19 infections. Pfizer is hoping that the phase 3 study will provide more information on these cases. This will allow for more accurate predictions of the potential impact the vaccine could make, says Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy.

Side effects of aches and fevers are already being observed in the study participants. However, researchers are optimistic because these side effects are comparable to standard adult vaccines but are worse than some of Pfizer’s other vaccines for the flu and pneumonia.

In efforts to keep with guidance from the Food and Drug Administration, Pfizer will not file for an emergency use authorization to distribute the vaccine until at least half of the patients in their study have been observed for two months, looking for any potential side effects. Pfizer is hoping to reach this threshold in the third week of November.

If authorized by the FDA, the initial supply of the vaccine will be limited. Pfizer says up to 50 million doses could be available globally by the end of the year and 1.3 billion available in 2021. This low number of initial vaccines available is due to the expected distribution challenges. The vaccine must be stored in frigid temperatures, thus making it a challenge to deliver to most places.