Researchers have developed an experimental vaccine that can provide broad protection against all 20 known strains of influenza A and B in animal tests, said an U.S. study published on Thursday.

The technique, messenger RNA, has been widely adopted for COVID-19 vaccines. It wraps RNA transcripts in packages that can be delivered into our body to teach our cells how to produce antibodies specific to the virus, like a messenger.

Influenza viruses are constantly evolving, making them a moving target for vaccine developers, The World Health Organization would announce a circulating strain each year based on experts' prediction. However, researchers sometimes get the prediction wrong, meaning the vaccine is less effective than it should be in those years. 

Hence, a universal vaccine would not mean an end to flu seasons, but would replace the guess work that goes into developing annual shots months ahead of the flu season each year.

The new multivalent vaccine is a nucleoside-modified vaccine that delivers tiny lipid particles containing mRNA instructions for cells to create replicas of so-called hemagglutinin proteins that appear on all 20 known influenza A virus subtypes and influenza B virus lineages, according to the report published in the journal Science.

In lab experiments, vaccinated animals' immune systems recognized the hemagglutinin proteins and defended against 18 different strains of influenza A and two strains of influenza B. Antibody levels induced by the vaccine remained unchanged for at least four months, according to the report.

The vaccine reduced signs of illness and protected from death even when the ferrets were exposed to a different type of flu not in the vaccine, the researchers said.

The universal flu vaccine, if successful in human trials, would not necessarily prevent infection. The goal is to provide durable protection against severe disease and death, study leader Scott Hensley of the Perelman School of Medicine at the University of Pennsylvania said in a statement.

"The idea here is to have a vaccine that will give people a baseline level of immune memory to diverse flu strains, so that there will be far less disease and death when the next flu pandemic occurs," he said.

However, questions remain regarding how to judge efficacy and potential regulatory requirements for a vaccine against possible future viruses that are not currently circulating, Alyson Kelvin and Darryl Falzarano of the University of Saskatchewan in Canada, wrote in a commentary published with the study.

While the promising results with the new vaccine "suggest a protective capacity against all subtypes of influenza viruses, we cannot be sure until clinical trials in volunteers are done," Adolfo García-Sastrem, director of the Institute for Global Health and Emerging Pathogens at Mount Sinai Hospital in New York, said in a statement.

(With input from Reuters)