The race is on for a coronavirus vaccine, and a few candidates are nearing their final laps.

COVID-19, the disease caused by the coronavirus, has killed more than 700,000 people since the World Health Organization was alerted to a cluster of atypical pneumonia cases in Wuhan, China, in December 2019.

Many experts warn that life won’t return to “normal” until a vaccine is widely available. Studies of potential candidates are moving faster than ever before, and some drugmakers believe a vaccine will be available to the public in 2021.

The average timeline to move from preclinical trials to regulatory approval is about 10 years for a vaccine, according to Brittany Kmush, an assistant professor who specializes in infectious diseases and vaccines at Syracuse University in New York.

“So we’re making great progress and definitely going quickly,” she says.

Despite the speed, vaccine researchers have remained adamant that they are not compromising on safety. Side effects from the potential vaccines documented thus far have included headaches and irritation at the injection site, which are relatively common for any vaccine.

“Vaccines are highly regulated to make sure they reach a very high bar of safety,” Kmush says. “As far as I know, nobody has been cutting those safety corners.”

But questions were raised about liability following a report from Reuters that most countries who have made a deal with British drugmaker AstraZeneca granted the company protection from future liability claims related to its COVID-19 vaccine. The U.S., however, already prevents tort claims on products that help control a public-health crisis under the 2005 Public Readiness and Emergency Preparedness Act.

Once a vaccine candidate is proven safe, then it must be tested for efficacy, or how well it actually prevents the disease. Typically, half the people in a trial will get the vaccine while the other half get a placebo. If the number of virus cases is lower in the vaccine group compared to the placebo group, then researchers can determine its efficacy.

Through Operation Warp Speed, the government’s multiagency effort to develop and deliver a coronavirus vaccine, the U.S. has invested in a handful of candidates to increase the chances that the U.S. “will have at least one safe, effective vaccine by 2021,” Department of Health and Human Services Secretary Alex Azar said in early August.

Roughly 140 coronavirus vaccines are currently in development globally, according to WHO, and more than 25 have started clinical trials. Amid reports of advanced testing progress in countries like China and Russia, the Trump administration has made vague pledges to work with any country that develops the first successful vaccine. But it remains unclear what that cooperation would look like and how any potential vaccine from those places would be secured and distributed in the U.S.

The U.S. government has shown interest in some vaccine candidates that have launched their final phase three trials after producing promising results in human testing so far.

This vaccine, which is being developed in the U.K. in partnership with AstraZeneca, generated an immune response in healthy participants aged 18 to 55 during an early trial. Researchers said they found no safety concerns with the vaccine, which produced its strongest immune response after two doses were administered to patients.

It works through non-replicating viral vectors, which means the vaccine doesn’t contain the actual coronavirus but rather a dead or weakened virus that has genetic material from the coronavirus.

The vaccine is in its phase three trial, which is its largest and last test. If the drug proves safe and effective, Oxford and AstraZeneca can seek regulatory approval. AstraZeneca said it plans to produce 2 billion doses on a non-profit basis. It hopes to have the vaccine ready early next year.

The U.S. has announced it will pay $1.2 billion for access to 300 million doses.

Leading infectious disease expert Anthony Fauci in July said that the progression from sequencing the coronavirus to getting Moderna’s potential vaccine into its phase three trial “is the best we, in the United States, have ever done.” The final test for Moderna’s vaccine in partnership with the National Institutes of Health has begun and aims to enroll about 30,000 people from 89 widely dispersed sites across the U.S. by the end of September.

Data from the trial should be available in November or December. The company announced in July that healthy adults who were given the vaccine in the phase one trial tolerated it generally well and all created neutralizing antibodies in response.

This vaccine method, referred to as messenger RNA, uses genetic material copied from the virus to mimic its infection and generate an immune response. Moderna’s vaccine would be the first to use mRNA in humans, if successful. The company announced in August that it plans to price the vaccine under $40 per dose in most cases. Moderna President Stephen Hoge told Congress in July that he would not promise to sell the vaccine at-cost.

The U.S. has given the company nearly $1 billion to support its vaccine efforts.

Pfizer is developing several vaccine candidates with German company BioNTech and the Chinese drug maker Fosun Pharma. One of them, BNT162b2, entered a combined phase two/three trial last month. Like Moderna, it uses mRNA.

The U.S. has agreed to spend nearly $2 billion for 100 million doses of the companies’ potential vaccine with the chance to acquire up to 500 million additional doses. Pfizer, however, did not take any money from the government to fund the development of the vaccine. HHS said the vaccines would be delivered at no cost to Americans, but the company did not commit to selling the vaccine at-cost to Congress in July.

Results from its phase one/two trials showed an immune response in volunteers.