LONDON — Scientists at Oxford University say their experimental coronavirus vaccine has been shown in an early trial to prompt a protective immune response in hundreds of people who got the shot.

British researchers first began testing the vaccine in April in about 1,000 people, half of whom got the experimental vaccine. Such early trials are designed to evaluate safety and see what kind of immune response was provoked, but can’t tell if the vaccine truly protects.

In research published Monday in the journal Lancet, scientists said that they found their experimental COVID-19 vaccine produced a dual immune response in people aged 18 to 55 that lasted at least two months after they were immunized.

“We are seeing good immune response in almost everybody,” said Dr. Adrian Hill, director of the Jenner Institute at Oxford University. “What this vaccine does particularly well is trigger both arms of the immune system,” he said.

Hill said that neutralizing antibodies are produced — molecules which are key to blocking infection. In addition, the vaccine also causes a reaction in the body’s T-cells, which help by destroying cells that have been taken over by the virus.

The experimental COVID-19 vaccine caused minor side effects like fever, chills and muscle pain more often than in those who got a control meningitis vaccine.

Hill said that larger trials evaluating the vaccine’s effectiveness, involving about 10,000 people in the U.K. as well as participants in South Africa and Brazil are still underway. Another trial is slated to start in the U.S. soon, aiming to enroll about 30,000 people.

How quickly scientists are able to determine the vaccine’s effectiveness will depend largely on how much more transmission there is, but Hill estimated that if there were enough data to prove the vaccine’s efficacy, immunization of some high-risk groups in Britain could begin in December.

He said the vaccine seemed to produce a comparable level of antibodies to those produced by people who recovered from a COVID-19 infection and hoped that the T-cell response would provide even more protection.

“There’s increasing evidence that having a T-cell response as well as antibodies could be very important in controlling COVID-19,” Hill said. He suggested the immune response might be boosted after a second dose; in a small number of people, their trial tested two doses administered about four weeks apart.

Hill said Oxford’s vaccine is designed to reduce disease and transmission. It uses a harmless virus — a chimpanzee cold virus, engineered so it can’t spread — to carry the coronavirus’ spike protein into the body, which should trigger an immune response.

Hill said Oxford has partnered with drugmaker AstraZeneca to produce their vaccine globally, and that the company has already committed to making 2 billion doses.

“Even 2 billion doses may not be enough,” he said, pointing to the ongoing surge of infections worldwide. “I think its going to be very difficult to control this pandemic without a vaccine.”

Hill said researchers were also considering conducting a “challenge” trial by the end of 2020, an ethically controversial test where participants would be deliberately infected with COVID-19 after being immunized to see if the vaccine is effective.

“This has been done before in 19 different infectious diseases to develop vaccines and drugs and is likely to happen for COVID-19 as well,” he said.

Numerous countries including Germany, France, the Netherlands, Italy, U.S. and the U.K. have all signed deals to receive hundreds of millions of doses of the Oxford vaccine — which has not yet been licensed — with the first deliveries scheduled for the fall.

Chinese researchers also published a study on their experimental COVID-19 vaccine in the Lancet on Monday, using a similar technique as the Oxford scientists. They reported that in their study of about 500 people, an immune response was detected in those who were immunized, although they couldn’t tell if people were protected from the disease because they weren’t exposed to COVID-19 afterwards.

CanSino Biologics’ vaccine is made similarly to Oxford’s except the Chinese shot is made with a human cold virus, and the study showed people whose bodies recognized it didn’t get as much of the presumed COVID-19 benefit. Still, China’s government already gave special approval for the military to use CanSino’s vaccine while it explores final-stage studies.

In an accompanying editorial, Naor Bar-Zeev and William Moss of the Johns Hopkins Bloomberg School of Public Health called both the Oxford and Chinese results “encouraging” but said further judgment should wait until the vaccine is tested on much bigger populations.

The World Health Organization’s director-general, Tedros Adhanom Ghebreyesus, called for any eventual COVID-19 shots to be fairly distributed globally, saying it was up to political leaders to ensure the most vulnerable populations also get immunized.

“But one of the worrying patterns we see is some countries moving in the other direction,” he warned.

Britain announced Monday it had secured access to another 90 million experimental COVID-19 vaccines made by Pfizer and others, a move some campaigners warned could worsen a global scramble by rich countries to hoard the world’s limited supply of COVID-19 vaccines.

Last week, American researchers announced that the first COVID-19 vaccine tested there boosted people’s immune systems just as scientists had hoped and the shots will now enter the final phase of testing. That vaccine, developed by the National Institutes of Health and Moderna, produced the molecules key to blocking infection in volunteers who got it, at levels comparable to people who survived a COVID-19 infection.

The vaccine being developed by Pfizer also works to trigger a similar dual immune response as the Oxford shot. Pfizer and its German partner BioNTech also released an encouraging early report Monday.

Nearly two dozen potential vaccines are in various stages of human testing worldwide, with a handful entering necessary late-stage testing to prove effectiveness.

AP Medical Writer Lauran Neergaard in Alexandria, Virginia, and Jamey Keaten in Geneva, contributed to this report.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.