The updated coronaviruses vaccine is Moderna's lead candidate to serve as a U.S. booster shot in the fall.

A booster dose combining the original vaccine with one that specifically targeted Omicron was tested by researchers. The combination produced 1.75 times the amount of neutralizing antibodies against Omicron as the existing Moderna vaccine did alone.

While those results may seem encouraging on their face, many experts worry that the virus is evolving so quickly that it is out pacing the ability to modify vaccines, at least as long as the US relies on human clinical trials for results.

Moderna found that the updated vaccine produced a stronger immune response than the existing vaccine a month after it was given. There were three earlier shots of Moderna's vaccine.

Some vaccine experts say that the most important thing right now is how well a new booster would protect against the latest subvariants, BA.4 and BA.5. 13 percent of new cases in the United States are attributed to the two subvariants, which were first detected in South Africa. They could out compete two other Omicron subvariants within a month.

The updated vaccine did not work against BA.4. The president of the firm said that researchers were still gathering data on those and other subvariants.

The levels of neutralizing antibodies triggered by the updated vaccine were two to threefold lower against the BA.4 and BA.5 subvariants, compared to those triggered against Omicron, according to a small sample. He said that the levels were still a very comfortable place.

Moderna officials are not sure if the reconfigured vaccine will offer more lasting protection than the existing one, but they are hopeful that it will.

The newer versions of Omicron seem to spread more quickly and may be better at evading the immune system. They are not certain if they cause more severe disease. The chief medical adviser to the White House said in an interview on Tuesday that there had been a slight increase in hospitalizations in South Africa.

Some vaccine experts say it makes sense to target the most recent versions of the virus rather than the forms that have already been overtaken.

Moderna and Pfizer, the makers of the other main coronaviruses vaccine in the United States, don't have enough time to run more human clinical trials and still make shots before the fall, when the Biden administration hopes to be able to offer an updated vaccine to counter.

It could force regulators to use data from laboratory tests and trials involving animals instead of robust human trials. It is possible that there will be a new variant or subvariant of concern by the fall.

The Food and Drug Administration's outside advisers are scheduled to meet on June 28 to discuss which vaccine formula would work best as a fall booster.

The final decision is always left to the F.D.A. The F.D.A. will probably keep as many irons as possible. They may need to use laboratory data and possibly animal data as alternative pathways of decision.

He said that people who are very concerned about protecting themselves will accept a booster with no long human trials.

A different model for coronaviruses vaccine development, like the one used to modify the flu vaccine every year, would be comfortable for many health care professionals. The composition of the annual flu vaccine is changed to meet new variant with minimal human tests.

The trial of the vaccine targeting Omicron started in February. The participants were all older than 57. After receiving two shots of Moderna's existing vaccine, all volunteers were given a booster dose eight months later.

437 volunteers received the booster designed to work against Omicron after the first booster, while 377 received a second booster. The updated booster produced a stronger immune response among those who had been exposed to the virus and those who hadn't.

According to data released by Moderna, the people who got the updated booster had a higher level of neutralizing antibodies than the people who got the existing booster.

Antibodies are the first line of defense against infections from the coronaviruses. The tests that defend against Covid-19 are more complex and time consuming.

The results were described as encouraging by Dr. Paul Burton. He said that this is a turning point in the fight against the virus because they can adapt to a variant.

A less than twofold increase in neutralizing antibodies over the existing vaccine is only a small benefit, according to Dr. Moore.

He asked if it was justified to switch vaccine composition given the cost and the logistical aspects of it. The argument will be about that.

Results from Pfizer and BioNTech's vaccine testing are expected soon.

Moderna released preliminary results on a vaccine retooled to attack theBeta variant. The firm said that that version of the vaccine caused a stronger immune response than the initial one. Moderna officials said the added protection against Omicron lasted for six months, but they thought an Omicron-specific vaccine would be a better choice.