With pharmaceutical company AstraZeneca's announcement Monday that its vaccine successfully prevented coronavirus infection, three candidates appear to be promising vital tools to curtail the COVID-19 pandemic.
Biotech firm Moderna and drug company partners Pfizer and BioNTech announced last week that their vaccines were ready to submit to the U.S. Food and Drug Administration (FDA) for emergency use authorization.
They are signs of hope as the global death toll from COVID-19 nears 1.4 million people, according to Johns Hopkins University.
However, scientists caution that all they know about these vaccines is what the companies have said in press releases.
Like movie trailers, "They provide some exciting scenes but leave a lot unsaid. You have to go see the whole movie," said Vanderbilt University infectious diseases professor William Schaffner.
More data will be available in the coming weeks, when the companies take their applications to the FDA. Until then, here is how the vaccines compare, based on the limited information presented in company press releases.
All three vaccines appear extremely effective.
The FDA told companies their products would have to be better than 50% effective to get emergency approval. All three far surpassed that mark while requiring two doses for maximum effectiveness.
Pfizer and Moderna both reported about 95% efficacy in their clinical trials.
The AstraZeneca vaccine was up to 90% effective, although one dosing regimen was less effective and reached only 62%.
However, the companies have not yet released all the data on how well the vaccine works in different age or ethnic groups, Schaffner notes, or for people with different medical conditions. The studies may be too small to answer those questions fully.
"The question is, effective in whom?" said Paul Offit, Director of the Vaccine Education Center at the Children's Hospital of Philadelphia.
Studies have not yet been done on children or pregnant women, Schaffner noted.
The studies also will not determine how long protection lasts.
And they will not say whether the vaccine prevents infection, or just lowers the amount of virus enough to keep a person from getting sick.
If vaccinated people still can carry and spread the virus, "you still have to maintain mask wearing and social distancing et cetera," Schaffner said, "which will make many people grumpy."
None of the vaccine companies have reported any major safety problems.
For Pfizer's and Moderna's vaccines, the most common side effects were sore arms lasting more than a day, fevers and fatigue.
"They're not at all what we call serious, but they're notable," Schaffner said. "That's much more than you get with flu vaccine."
AstraZeneca said no serious safety problems have been identified but has not released details.
The company paused the trial twice because two study participants developed serious neurological problems.
The study's safety board said they were coincidental and not because of the vaccine, but outside experts have not yet seen the data.
"You'd like to see all that information, and we don't have that information," Offit said.
AstraZeneca may have the most doses available early.
CEO Pascal Soriot did not specify exact numbers but said the company will have "hundreds of millions of doses on approval." The company has agreements to produce 1.7 billion doses worldwide, including a deal with the Serum Institute of India to produce 1 billion doses mainly for low- and middle-income countries.
Pfizer says it will produce 50 million doses worldwide by the end of this year and up to 1.3 billion doses next year.
Moderna aims to ship 20 million doses in the United States this year and 500 million to 1 billion doses globally in 2021.
All three companies have taken the extremely unusual step of scaling up manufacturing before the results of their clinical trials were in. That means doses can start being distributed as soon as regulators give the green light.
Moderna and AstraZeneca did so with government funding. Pfizer had government purchase guarantees.
AstraZeneca's vaccine is the easiest to ship and store. It lasts for at least 6 months in the refrigerator.
Moderna's and Pfizer's vaccines need to be frozen for long-term storage. Moderna's lasts at least a month in the refrigerator. Pfizer's needs special ultra-cold freezers that are not commonly available outside academic medical centers and lasts up to five days in the refrigerator.
AstraZeneca's is cheapest.
The company has pledged not to make a profit on the vaccine during the pandemic. Shots are expected to cost under $5 each, compared with around $20 to $40 for the other vaccines, according to news reports.
Since governments will be the main buyers, cost will be a factor mainly in low- and middle-income countries, and with nonprofit and public-private groups who will be purchasing and distributing the vaccines.