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…

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.”…

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.