The mRNA vaccines by pharmaceutical giants Pfizer and Moderna appeared to give the highest boost of antibodies after 28 days of the extra dose, though other vaccines in the study may take more time to build up better immunity.

After the United States greenlit “mix-and-match” boosters, the Biden administration is now encouraging more people to get extra doses as scientists race to understand the newly-identified omicron variant which spurred many countries to tighten restrictions.

The new research on safety and immune response, published Thursday in British journal the Lancet, looks at how to “optimize selection of booster vaccines” by examining people who initially took two doses of either the Pfizer-BioNTech or the Oxford-AstraZeneca shot. The United States has not authorized the Oxford-AstraZeneca shot.

The inoculated participants got a range of boosters, including the experimental shot from the company CureVac that was pulled after less effective results. The others included Johnson & Johnson, Moderna, AstraZeneca and Pfizer-BioNTech, as well as the Novavax and Valneva vaccines which are under review in Europe.

Side effects included fatigue and headache, but serious reactions were not common with any of the boosters, the report said.

The research, led by scientists at the University of Southampton, measured responses that can help predict the level of immunity such as higher antibody levels and the creation of T cells.

Almost all the vaccines in the study bolstered those responses regardless of the initial doses, it said. One exception was the combination of an extra dose from Valneva in people who had first received Pfizer shots, which did not meet benchmarks for a “clinically important difference.”

All seven vaccines appeared to offer more immunity after two AstraZeneca doses and six were effective after two Pfizer shots “with no safety concerns” though with varying results, according to the researchers.

While the two mRNA vaccines produced the biggest antibody boost at 28 days, “the vaccines work in different ways,” infectious diseases professor Saul Faust of the U.K. National Institute for Health Research, who led the trial, told The Washington Post on Friday. “The other vaccines are . . . still a massive boost compared to what was already protecting us,” he said.

The researchers noted that more work will be necessary as well to understand the relationship between immune response after coronavirus vaccines and longer-term protection against disease.

The results come after Britain expanded its rollout of boosters, offering all adults the prospect of an extra dose, as biotech firms examine how their existing shots fight the latest variant and how to adapt.

The spread of omicron has raised more warnings about vaccine inequity around the world, with many people in poorer countries yet to receive their first shot.

“We’ve got just as much responsibility” to overcome hesitancy in many countries and get enough vaccines to people who have less access, Faust said. The latest findings also show that “countries can reach for what they have available.”

To help with vaccine supplies, the study suggests that half a dose of Pfizer-BioNTech could have an effective response as a booster. The message behind the mRNA vaccines quickly producing a high antibody level “is that we might be able to boost more people with the same amount of vaccines in the future,” he added. “It’s exciting for global deployment that we might be able to reduce the dose and get just as good immunity.”