Prior to the coronavirus crisis, the fastest a vaccine had ever been developed was back in the 1960s, to prevent mumps. That took four years. The race to come up with a vaccine against SARS-CoV-2, the virus that causes COVID-19, has been hailed as a historical milestone for undergoing development in under a single year. Never before has a vaccine been produced so quickly.

Americans largely appear to welcome these efforts polling suggests that most adults will get a coronavirus vaccine when one becomes available to them. But its worth asking exactly how scientists were able to produce multiple vaccines in such a short time span.

HuffPost spoke to three experts about the speed at which vaccines against the coronavirus have been developed: Dr. Florian Krammer, a microbiology professor at Mt. Sinais Icahn School of Medicine in New York City; Dr. Gigi Gronvall, a senior scholar and associate professor at Johns Hopkins; and Dr. Thad Stappenbeck, a leading physician at the Cleveland Clinic. All of them said they understood why people may have questions about the process but agreed that the first doses rolling out are safe and effective and all of them indicated they would get the shots themselves, particularly as the risk of contracting COVID-19 compounds over time.

Heres how scientists broke records to arrive at a vaccine.

Funding is usually a big hurdle that slows down vaccine development. Not so during the coronavirus pandemic.

Billions of vaccine doses are going to be required to keep the virus in check over the next several years after it crippled the global economy and prompted unemployment to rise to levels not seen since the Great Depression. That enormous market and unprecedented demand prompted ample pledges for funding for its development.

Typically, when you produce or develop a vaccine, at each step along the road, you ask yourself, do we have the money to move forward? Is it worth moving forward? Krammer told HuffPost.

The U.S. government alone has pledged more than $10 billion to companies working on coronavirus vaccines as part of Operation Warp Speed, the Trump administrations effort to support vaccine developers. Drugmakers are also more willing to take financial risks given the urgency of the crisis.

These companies were able to let the science proceed, Gronvall said.

The trial phases and manufacturing overlapped somewhat to save time without compromising safety.

The process for developing vaccines is standardized: Normally, an exploratory and design period can take several years before eventually leading to clinical trials, which are held in three phases, followed by regulatory review and large-scale manufacturing. (A helpful visualization can be found in Nature .) Researchers aim to establish an appropriate dosage level in Phase I trials, moving on to a larger group of human study subjects in Phase II to show hopefully that the drug offers some health benefit.

Last springs Phase I trials initially saw a whopping immunologic response, Stappenbeck said, so the FDA let them essentially move right into Phase II, which was also successful.

Phase III is the big one, where a large number of people is recruited to try out the drug (or a placebo) and evaluate the side effects. Phase III ends when enough people have contracted the virus in the vaccine group compared to the placebo group, giving you the vaccines efficacy.

With many other diseases, Krammer explained, it takes a while to build up enough cases in the pool of study participants.

Now we are in this absurd situation that, because there is so much virus, we get results quicker, he said. If you would have controlled the virus circulation very well, and there would have been no wave in the fall, we would not have any efficacy results. We would still be waiting.

Usually companies developing vaccines aim to mitigate financial risk by waiting until each phase is complete before moving on to the next one. Companies developing coronavirus vaccines, however, have been able to move more quickly because the financial risk is lower.

They literally did everything that they would have done over a period of years in 10 months. Think of it as an accordion thats pushed in, so everything is there, its just in a smaller package, Stappenbeck said.

In the case of Pfizer, British Medicines and Healthcare Products Regulatory Agency sped things along further by looking at thousands of pages of safety and efficacy documentation on a rolling basis, whereas the American Food and Drug Administration waited for more complete data . Officials were eating turkey sandwiches on Thanksgiving while reviewing documents, the FDAs Peter Marks, who leads the group that decides which coronavirus vaccines are safe enough for widespread use, said in an interview with the Journal of the American Medical Association. Drugmakers have also tried to be transparent in their research by making a lot of their documentation freely available online, Krammer said, so people can look at it for themselves.

A number of problems can crop up at any point along the entire process. Sometimes researchers run into unexpected side effects in humans, or safety and efficacy problems, or trouble with regulatory agencies. A vaccine being developed by Australias University of Queensland became the first to be abandoned earlier this month after researchers discovered that it produced false-positive HIV test results.

In normal times, a drugmaker would certainly not get started on large-scale manufacturing during the clinical trial phases, because relatively few drugs actually get approved each year.

You wouldnt put up that kind of money, Gronvall said.

But thats what companies have been doing. Moderna, whose vaccine is close to cinching emergency approval for use in the U.S., started scaling up its manufacturing abilities in late March and struck a deal to provide the U.S. 100 million doses in August. Pfizer, whose vaccine was the first to secure emergency approval in both the U.K. and the U.S., settled on which facilities it would use to make its drug back in May. By the end of July, the company had agreed to supply the U.S. with an initial order of 100 million doses, even though final clinical trial results were several months away.

Scientists werent starting from scratch. They had research to build on from past viral outbreaks.

The last two decades have seen two other outbreaks of different coronaviruses. (Technically, the term coronavirus refers to a whole family of viruses, but it is often used as shorthand for the specific virus that causes COVID-19 due to its sheer prevalence.)

SARS-CoV (severe acute respiratory syndrome coronavirus) and MERS-CoV (Middle East respiratory syndrome coronavirus) emerged in 2002 and 2012, respectively. Scientists began the process of developing a vaccine for SARS-CoV, and two drugs underwent Phase I trials, Krammer describes in a paper published in the journal Nature . Development stopped, though, because the virus had petered out by 2004 and it was no longer worth the cost. A vaccine for MERS-CoV is still being developed, eight years after it first appeared. Neither virus, of course, came anywhere near the scale of the current crisis in the U.S., just eight people caught SARS and two people caught MERS.

Because the virus currently spreading rapidly around the world, SARS-CoV-2, is related to SARS-CoV and MERS-CoV, researchers had a head start on how to potentially design the vaccine. They already knew which protein to focus on.

Plus, they had a body of research on mRNA vaccines.

In another scientific milestone, both the Pfizer and Moderna vaccines successfully make use of mRNA, or messenger RNA, which has never before been used in a vaccine approved in the U.S.

That doesnt mean its a brand-new technology.

MRNA vaccines have been studied for more than a decade, with the first human trials beginning in 2013. So while we may only have data going back two months on the people who took, for example, the Pfizer vaccine, scientists have been tracking people who have taken experimental mRNA drugs for much longer.

There is certainly data in, you know, not in tens of thousands of people, but in hundreds of people where you have long-term data, Krammer said.

Whereas many vaccines protect us by putting inactivated virus in our bodies, triggering an immune response that produces antibodies, a messenger RNA vaccine works differently. It contains instructions that teach our cells how to make a small piece of the virus in this case the spike protein unique to SARS-CoV-2 which also triggers an immune response. Our bodies then know how to recognize that spike protein and fight off the virus if we are exposed to it for real. The mRNA does not interact with our human DNA whatsoever.

RNA itself is very delicate, which is why some of the vaccines need to be kept at extremely low temperatures in special freezers. The instructions are encased in a lipid, or fat, for transport.

They had developed a really clever technology where they were able to take these really unstable mRNA molecules, stabilize them enough, and then encapsulate them, Stappenbeck said.

A lot of the work that would have taken years and years and years to develop to do in animal models had essentially already been done.

Once the instructions have been delivered, the elements of the vaccine get broken down right away.

The design of it is really simple. Its just an mRNA and the lipid. Theres really no other chemicals, no other factors that are present in it that could produce side effects in some small group of population, Stappenbeck said.

The vaccine effort is global.

Chinese scientists sequenced the viruss genome back in January, sharing the information with the world soon after. Independent regulatory agencies in the U.S., Canada, Britain and elsewhere have been largely open and collaborative with one another to move the process along more efficiently than ever before.

Although we have seen the Trump administration try and exert undue influence over the CDC, none of the experts HuffPost spoke to had any concern that the administration would affect the FDAs science-bound decision-making process.

Krammer stressed that the FDA is widely respected, saying, Theres a lot of good scientists there. In the end, they are data-driven. Theyre not driven by politics, and they know what they have to lose.

The other thing is that scientists and physicians around the world have read the same reports, Stappenbeck said. What you dont see is physicians from the Cleveland Clinic getting on Twitter, or blogs, or interviews, and saying that theyre wrong.

If people are really concerned about this, all they have to do is, you know, Google COVID-19 Moderna and then whatever major medical center they want to put in, and they can read about what the experts at that medical center are saying, he added. And what theyll see is a uniform chorus, really across the country, where everyone is in agreement that this is the right thing to do.

As with any medical intervention, there will always be some risk to take into account.

But you dont need to be an expert to evaluate it, Krammer told HuffPost.

Just thinking about risk analysis, what do I risk if I do take the vaccine versus what do I risk if I dont take the vaccine? If I do take the vaccine, I know that 40,000 people were fine, he said, referring to the total number of people who received a vaccine in Pfizer and Modernas clinical trials. On the other side, you have the risk of getting infected.

Negative side effects generally happen relatively soon after vaccination. In 2009, for example, an increased risk of narcolepsy was discovered shortly after people in Europe began taking an H1N1 vaccine made by GlaxoSmithKline. (That drug was never authorized for use in the U.S.) Experts told HuffPost that it is possible some previously unrecorded low-frequency side effects will become apparent in the next several months as more and more people get the new vaccines. That is largely because data is lacking for some groups, such as pregnant women, who are generally advised to hold off on getting the vaccine. People with severe allergies, too, are being told to wait.

It not likely that side effects will crop up unexpectedly years down the line.

Medical centers that dole out the vaccine in the U.S. are also working to monitor people who have received the drugs for any adverse effects. The CDC is using a smartphone app called V-Safe , which allows people to complete health check-ins, to help them do so, too.

People who get a coronavirus vaccine are likely to experience some of the more common side effects in the following day or so. We should anticipate potentially feeling fatigued or experiencing headache, chills or muscle pain shortly after getting the jab but know that is to be expected.

To learn more about side effects of the coronavirus vaccines, head here .