Drug manufacturers are trying to develop a vaccine for the new coronavirus as quickly as possible in an effort to slow its spread.

But scientists and medical experts are concerned that moving too quickly to release a vaccine for the virus could create added risk.

Studies suggest that coronavirus vaccines carry the risk of what is known as vaccine enhancement. This is a condition in which the vaccine can make the disease worse when a vaccinated person is infected with the virus.

Scientists do not know what causes vaccine enhancement, but it has prevented the successful development of a coronavirus vaccine.

There are many coronaviruses, such as the one responsiblefor Severe Acute Respiratory Syndrome, in 2003,also known as SARS. Other coronaviruses have been linked to Middle East Respiratory Syndrome, known as MERS, and even the common cold.

COVID-19 is the name of the disease resulting from infection with the new coronavirus.

Normally, researchers need months of tests to study the possibility of vaccine enhancement in animals. But the Reuters news agency reports that some drug-makers are moving forward with limited human testing because of the new coronavirus outbreak.

Peter Hotez is head of the National School of Tropical Medicine at Baylor College of Medicine in Texas.

Hotez told Reuters that he understands the need to speed up development of a vaccine. But he added, "...from everything I know, this is not the vaccine to do it with."

Hotez worked on the development of a vaccine for the SARS virus in the early 2000s. He said researchers at the time found that some vaccinated animals developed more severe disease compared to unvaccinated animals when they became infected.

"The way you reduce that risk is first you show it does not occur in laboratory animals," he added.

Hotez spoke this month to a United States congressional committee about the need to provide public money for vaccine research. There are no vaccines for any of the coronaviruses that have caused outbreaks in the past 20 years, Reuters noted.

But health experts believe at this time that speeding vaccine development is worth taking the risk.

The World Health Organization (WHO) called a meeting last month to organize efforts to fight the new coronavirus. The meeting was not open to the public.

At the conference, scientists representing government-supported research groups and drug-makers agreed that the threat was severe. Four people attending the meeting told Reuters that vaccine developers agreed that efforts should move forward quickly to human tests before animal testing was completed.

Marie-Paule Kieny is a doctor and a former official with the WHO. She said that moving forward quickly can create new risks. "You have to balance this with the risk that you impose on a small number of people, and do all you can do to mitigate this risk as much as possible," she told Reuters.

Testing moving forward

Researchers and drug companies are developing at least 20 possible vaccines to help protect against the new coronavirus.

The biotechnology company Moderna is working with the U.S. government-supported National Institutes of Health (NIH) on one vaccine candidate. It is said to be closest to human testing.

Moderna announced a trial with 45 people in Seattle, Washington this month. Since that area was chosen, it has had many cases of the new coronavirus, including more than 20 deaths.

The NIH told Reuters that testing on animals for vaccine enhancement will take place at the same time as human tests. The NIH said the trial is expected to last 14 months.

Another company, Inovio Pharmaceuticals, is developing a coronavirus vaccine in cooperation with a Chinese company. It is not waiting for tests on animals and expects to start testing on 30 U.S. volunteers in April.

Inovio chief Joseph Kim said, "The community as a whole weighed that and said we don't want to delay the clinical process."

Early warning signs

Earlier work on coronaviruses and other vaccines have drug developers on guard for warning signs.

The best-known example of vaccine enhancement took place in the United States in the 1960s. At the time, NIH researchers created a vaccine to fight a virus - called RSV - that caused lung infections in babies. But most of the babies who received the vaccine developed severe cases of the disease, and two youngsters died.

Research has shown that coronaviruses have the ability to produce this kind of reaction. But testing for the risk of vaccine enhancement takes time because scientists must develop genetically-engineered mice to test the vaccine. Development of such animals is just starting in several laboratories around the world.

Moderna, Inovio and several other companies are not waiting for that process to be completed. They want to carry out tests on humans in record time. The latest new coronavirus, which causes COVID-19, was first identified in late December.

Johnson & Johnson also is trying to create a vaccine. The company is developing animal models to test for vaccine enhancement. It hopes to begin human tests by October.

Johan Van Hoof, head of the company's vaccine division, said, "People know how traumatic the RSV experience was." He added "When you see signals in animals like this, we should not ignore them."

I'm Mario Ritter Jr.

And I'm Ashley Thompson.

Julie Steenhuysen reported this story for Reuters. Mario Ritter Jr. adapted it for VOA Learning English. George Grow was the editor.

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vaccine enhancement –n. when a vaccine causes an increase in a viral infection

impose –v. to cause something to affect a person using a form of authority

mitigate –v. to make something less harmful or severe

clinical –adj. related to treating real patients

traumatic –n. related to or causing serious problems or harm