SAN FRANCISCO -- Another COVID-19 vaccine -- AstraZeneca -- is poised to arrive in the U.S., bringing a fourth option for Americans. Each vaccine is administered slightly differently, some can result in different side effects, and studies have shown varying efficacy. We're breaking down the key differences between the COVID-19 vaccines available, plus the likely-to-be approved AstraZeneca.
The Pfizer, Moderna and Johnson & Johnson vaccines have already been authorized for emergency use by the FDA. AstraZeneca is expected to apply for emergency use authorization in the next few weeks.
Johnson & Johnson is the only one-shot vaccine. The others all require two doses at different time intervals. Pfizer suggests the two doses be administered three weeks apart, Moderna suggests four weeks, and AstraZeneca suggests four to 12 weeks apart.
In a clinical trial, Pfizer says its vaccine was 95% effective in preventing COVID-19 cases with symptoms and 100% effective when it came to preventing severe cases. Moderna cited similar results, with 94% efficacy.
Johnson & Johnson found its vaccine, which was tested in the U.S. amid a larger surge in COVID-19 transmission, was 72% effective overall and 86% effective in preventing severe disease.
The most recent U.S. trial shows AstraZeneca's vaccine was 79% effective at preventing symptomatic COVID-19 cases and was 100% effective in stopping severe cases and hospitalization.
A word of caution before you compare the above efficacy rates side-by-side: the vaccine clinical trials were done under different conditions, which could yield different results. For example, the Johnson & Johnson vaccine was tested in an environment with far more COVID-19 variants circulating and during a time when cases were surging. The "best" vaccine is the one you are offered, doctors continue to remind the public.
All three approved vaccines have the same common side effects, per the Centers for Disease Control:
Concern over blood clots prompted several European countries to halt use of the AstraZeneca vaccine. However, the European Medicines Agency has concluded, after an investigation, that the vaccine did not raise the overall risk of blood clots, but could not rule out that it was connected to two very rare types of clots.
In the most recent U.S. trial of 20,000 people, experts found no increased risk of blood clots.
According to the CDC, the U.S. has received about 77 million Pfizer doses, 75 million Moderna doses and 4 million J&J doses at publication time. Remember: the J&J vaccine only requires one dose, but it takes two Pfizer or Moderna doses to fully vaccinate one person.
At this point, no. It's possible choices may come down the line, if supply grows large enough.
The Associated Press contributed to this report.