3 coronavirus vaccines so far seem to prevent COVID-19. Here's how their efficacy compares to vaccines for flu, measles, and more.

• AstraZeneca reported last week that its coronavirus vaccine candidate was 70% effective, on average, in preventing COVID-19 in clinical trials.
• Moderna and Pfizer's vaccine candidates were found to be 94% and 95% effective, respectively.
• The chart below shows how the efficacy of those vaccine candidates compares to the effectiveness of shots for chickenpox, flu, measles, and polio.

Three coronavirus vaccine candidates now lead the pack.

Moderna, a Massachusetts-based biotech firm, released updated results from its late-stage clinical trial on Monday. The data so far show its vaccine is 94.1% effective at preventing COVID-19.

AstraZeneca and the University of Oxford announced their early trial results last week, which show their vaccine to be 70% effective on average. The week prior, Pfizer and BioNTech announced their vaccine had a 95% efficacy rate.

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 the three coronavirus vaccine frontrunners 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 the three 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 an analysis of 196 COVID-19 cases observed among 30,000 people in its phase 3 clinical trial. Some participants were given the company's experimental two-shot vaccine while others just got two placebo injections.

Out of those 196 cases, 185 were people who got placebo shots. Just 11 people who got COVID-19 had received Moderna's vaccine. The results also suggested that the vaccine prevented both mild COVID-19 and serious illness among participants. Researchers found 30 cases of severe COVID-19 among the placebo group and zero among those who got the vaccine.

Pfizer's results, meanwhile, were based on 170 cases of COVID-19 observed among more than 43,000 study participants. Its two-dose vaccine was found to prevent both mild and severe coronavirus infections, too.

AstraZeneca's trial results, based on 23,000 volunteers in Brazil and the UK, were more complicated. A dosing error during part of its trial meant that about 2,700 participants received a half-strength dose in their first shot, followed by a full-strength dose in their second. In that subgroup, the vaccine was 90% effective at preventing COVID-19, AstraZeneca said. But in a group of about 9,000 participants who got two full-strength doses, that efficacy rate was 62%.

The 70% figure is an average of the two.

It's still unclear whether any of the three vaccines prevents asymptomatic coronavirus infections, since trial participants were only tested for COVID-19 if they experienced symptoms.

A 2-dose regimen

All three of the leading vaccines require two shots. Pfizer's doses are given three weeks apart, while Moderna and AstraZeneca's are administered at least a month 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 are likely to require two shots as well. Novavax's phase 3 trial is testing two shots given three weeks apart. Johnson & Johnson, meanwhile, is testing both a single-dose and a two-dose vaccine in simultaneous phase 3 trials.

The coronavirus vaccine is likely to 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.

Moderna plans to file for emergency approval for its coronavirus vaccine on Monday, the company said in a statement. CEO Stephane Bancel said the company plans to produce 100 million doses — enough to vaccinate 50 million people — by the end of March 2021.

Pfizer and BioNTech asked the FDA to authorize their vaccine a week ago. Pfizer has said the bulk of the doses — up to 1.3 billion — will be ready sometime next year as well.

AstraZeneca, meanwhile, will likely run a second global trial, since the dosage error may have skewed its results, CEO Pascal Soriot said on Thursday. The company still hopes to distribute 3 billion doses in 2021, however.

None of the three frontrunners have published their late-stage clinical trial data yet or submitted findings to a peer-reviewed medical journal. So questions remain about how long protection will last and to what degree the vaccines can prevent the coronavirus from spreading.

This story has been updated with additional information. It was originally published November 9.

Andrew Dunn contributed reporting.