A healthcare worker injects a patient with the Pfizer and BioNTech coronavirus vaccine during the phase 3 trial in Turkey in October.

Pfizer and its German partner BioNTech announced Monday that their coronavirus vaccine candidate works to prevent COVID-19.

The partnership is first to report positive results from the final stage of clinical trials: The shot was found to be more than 90% effective in preventing COVID-19, based on 94 cases of the disease observed in an interim study.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, called that high percentage "just extraordinary."

The Food and Drug Administration has said any coronavirus shot must be at least 50% effective to get authorization. Most experts had hoped for 70% efficacy or higher.

Not all vaccines are equally effective. Some, like the seasonal flu vaccine, hover below 60%. Others, like the polio vaccine, are almost 100% effective. Here's how Pfizer's coronavirus vaccine candidate compares to four existing vaccines.

A two-dose regimen

The polio vaccine has been distributed for 65 years — plenty of time for scientists to build a thorough understanding of the vaccine's effectiveness.

By constrast, Pfizer's coronavirus vaccine candidate — and others from drugmakers like AstraZeneca and Moderna — have only been tested in clinical trials. So the preliminary results from the trial show how effective the vaccine could be given ideal circumstances in which everyone is given the shot. That measure is known as vaccine efficacy.

Pfizer's vaccine requires two doses: two shots three weeks apart.

Many established vaccines also require back-to-back doses to be most effective, including the shots for measles and varicella. Two doses of the MMR (measles, mumps, rubella) vaccine are 97% effective at protecting someone from measles, whereas a single dose is 93% effective.

Why the coronavirus vaccine will likely be more effective than a flu shot

Viruses mutate over time: As they replicate, minute errors are introduced into the virus' genetic code, and those can then spread through a virus' population. While most mutations are inconsequential, occasionally one can appear that undermines people's immunity. But the new coronavirus mutates slowly, which means a vaccine would most likely be effective long-term.

The flu, by contrast, mutates quickly, which is why a new vaccine is needed each year.

Pfizer still wants more data on its vaccine's safety before requesting emergency approval from the FDA. The company's clinical trial is not yet complete, and Pfizer has not published its data nor submitted findings to a peer-reviewed medical journal or regulators. So many questions remain.

Still,  the drugmaker has said the bulk of its doses — up to 1.3 billion — will be ready in 2021.

In a briefing on Monday, Fauci said Pfizer's success will "have a major impact on everything that we do with regard to COVID."

Andrew Dunn contributed reporting to this story.

Receive a daily news update on your cellphone. Or get the best of our site emailed to you.

Go to the Business Insider front page for more stories.