Virus-fighting antibodies capable of blocking the omicron variant persist four months after a third shot of the Pfizer-BioNTech coronavirus vaccine, according to a new study. 

The study, which was published on a preprint server Saturday, gives a first hint about the durability of vaccine protection, with a key line of immune defense remaining intact. The study has not yet been peer-reviewed and will need to be replicated and extended to a longer period.

The laboratory study suggests another shot may not be needed right away - a question that has caused anxiety for people wondering if and when they will need to get another booster.

"This is very, very new for the field," said Pei-Yong Shi, a microbiologist at the University of Texas Medical Branch in Galveston whose team tested antibodies in the blood of vaccinated people against an omicron-like virus in collaboration with Pfizer scientists. "That really shows that at least up to four months, post-dose three, there are still substantial neutralizing activity against omicron."

Antibodies are known to drop off in the months after vaccination. Omicron drew special concern because even in the month after two doses, when levels of virus-fighting antibodies should be near their peak, antibodies capable of blocking omicron were minimal, and even undetectable in many subjects. Those omicron-blocking antibodies were even less evident at the time people were about to receive a third dose.

Multiple studies have shown this erosion.

"You wait a month and check the blood and almost zilch neutralization activity against omicron," said Duane Wesemann, an immunologist at Brigham and Women's Hospital in Boston not involved in the study.

A third dose rebuilds that line of immune protection against omicron, but just as important, it increases the breadth of the immune response by creating a more potent repertoire of antibodies - a process known as affinity maturation. This greater breadth of protection is thought to be a large part of why the third shot is so helpful against omicron.

A major question had been how long that shored-up protection would last.

During the four months after a booster shot, antibodies against omicron did drop, the study found. But they remained high enough that, at least inferring from comparisons to other variants, they should continue to provide a layer of protection, Shi said.

The big uncertainty, said Shane Crotty, a vaccine expert at La Jolla Institute for Immunology, is whether antibody levels continue to drop after four months, or eventually stabilize at some level.

A study from the United Kingdom found that while protection against symptomatic infections is lower than for delta, even after a third dose, protection against hospitalization remains high. That study found that protection against hospitalization dropped from 92% in the month after the third dose to 83% for people at 10 or more weeks after that shot.

Israel began offering a fourth shot to people at risk for severe illness in December. But researchers announced at a recent news conference that while a fourth shot sends antibodies higher, it does not appear to provide greater protection against symptomatic infection.

Pfizer and BioNTech said in a statement Monday that they will soon be testing both a fourth dose of the original vaccine and an omicron-specific booster in human trials.

The companies plan to manufacture 4 billion doses of their vaccine this year, regardless of whether they continue to make the original shot or revise it to match omicron, the companies said.

Antibodies are just one easily measurable component of the immune response, and protection against severe illness and hospitalization can remain intact even as the levels of antibodies drops off.