Moderna and Pfizer's coronavirus vaccines are more than 90% effective. Here's how that compares to shots for the flu, measles, and more.

  • Moderna announced Monday that its coronavirus vaccine candidate was found to be 94.5% effective in preventing COVID-19 in clinical trials.
  • Pfizer announced last week that its coronavirus vaccine candidate was found to be more than 90% effective.
  • The chart below shows how those vaccine candidates' efficacy compares to the effectiveness of vaccines for chickenpox, flu, measles, and polio.
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Moderna, a Massachusetts-based biotech firm, announced Monday that trial results so far suggest its vaccine is 94.5% effective in preventing COVID-19. It's the second company to report positive findings from the final stage of clinical trials: Last week, Pfizer and BioNTech reported that their vaccine was found to be more than 90% effective.

"Now we have two vaccines that are really quite effective. I think this is a really strong step forward to where we want to be about getting control of this outbreak," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NBC's Today on Monday.

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 Moderna and Pfizer's coronavirus vaccine candidates compare to four existing vaccines.


How experts measure vaccine effectiveness 

The polio vaccine has been distributed for 65 years — plenty of time for scientists to build a thorough understanding of its effectiveness. That's a measure of how much it reduces the incidence of cases among people who get vaccinated relative to those who do not. 

The greater that reduction, the higher the vaccine's effectiveness.

Because Moderna and Pfizer's coronavirus vaccine candidates have only been tested in clinical trials however, their reports center on a measure known as vaccine efficacy. It's a calculation of how effective the vaccine could be given ideal circumstances in which everyone is given the shot.

Moderna's announcement was based on 95 cases of COVID-19 observed among the 30,000 participants in its phase 3 clinical trial. Among that larger group, some people were given the company's experimental vaccine while others just got a placebo injection.

Out of those 95 cases, 90 of them were people who'd gotten a placebo shot. Just five people who got COVID-19 had received Moderna's vaccine. The results showed that the vaccine not only prevented mild COVID-19 among participants, it also also appeared to prevent serious illness too. Researchers found 11 cases of severe COVID-19 among the placebo group and zero among those who got the vaccine.

Pfizer's vaccine, meanwhile was also found to prevent mild illness, but the company didn't reveal whether its shot prevented serious coronavirus infections, too.

It's also still unclear whether either vaccine prevents asymptomatic coronavirus infections, because trial participants were only tested for COVID-19 if they experienced symptoms.

A 2-dose regimen

Both Moderna and Pfizer's vaccines require two shots. Moderna's two doses are administered a month apart, while Pfizer's are given three weeks apart. 

Many other vaccines — including the one that protects against measles — also require back-to-back doses to be most effective. Two doses of the MMR (measles, mumps, rubella) vaccine are 97% effective at protecting someone from measles, whereas a single dose is 93% effective.

Other coronavirus vaccine candidates still in trials may require two shots as well, including AstraZeneca's; the company's phase 3 trial is testing two shots given four weeks apart. Johnson & Johnson, meanwhile, is testing both a single-dose and a two-dose vaccine in simultaneous phase 3 trials.

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 against any existing viral strains.

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

Both Pfizer and Moderna plan to request emergency approval for their coronavirus vaccines from the FDA in the new few weeks.

But neither company's clinical trial is complete yet, and neither has published their data or submitted findings to a peer-reviewed medical journal or regulators. So many questions remain.

Still, according to Moderna CEO Stephane Bancel, the company plans to produce 100 million doses — enough to vaccinate 50 million people — by the end of March 2021. Pfizer has said the bulk of its doses — up to 1.3 billion — will be ready sometime in 2021 as well.

Andrew Dunn contributed reporting to this story.

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