CAPE TOWN (NYTIMES) - A flurry of new laboratory studies indicate that vaccines, and especially booster shots, may offer protection against the worst outcomes from the fast-spreading Omicron coronavirus variant.

The highly mutated virus, however, will still cause many breakthrough infections in vaccinated people and in those who have been infected with older versions of the virus, according to the research.

At a World Health Organization meeting on Wednesday (Dec 15), scientists reported on several studies suggesting that T cells in vaccinated people can put up a strong defence against the variant, which could help prevent severe disease, hospitalisation and death.

Also on Wednesday, Dr Anthony Fauci, President Joe Biden's top medical adviser for the coronavirus response, shared preliminary data from his institute's analysis of the Moderna vaccine.

While two shots produced a negligible antibody response against Omicron in the laboratory, the protection shot up after a third dose, he said.

Other researchers at the WHO meeting presented similar results, showing that booster shots of either Moderna or Pfizer mRNA vaccines lifted antibodies back to levels believed high enough to offer strong protection against infection.

Though the research is based on preliminary observations of cells in the laboratory, it is nevertheless a welcome departure from a torrent of worrying new data about Omicron.

Over the past week, it has become increasingly clear that Omicron can deftly evade antibodies, part of the body's first line of defence, which probably explains why infections with the variant have exploded in many countries.

But antibodies are not the only important players in a person's immune response to the virus. T cells have their own role.

"The good news is that T cell responses are largely maintained to Omicron," said Dr Wendy Burgers of the University of Cape Town during a presentation of new research she and her colleagues have carried out in recent days.

Omicron infections are happening more frequently in two groups of people who carry antibodies: those who have received shots, as well as those who aren't vaccinated but have recovered from an earlier infection with the coronavirus.

This week, scientists in South Africa reported that two doses of the Pfizer vaccine were 33 per cent effective against an Omicron infection, down from about 80 per cent during what Dr Fauci called "the pre-Omicron era". The study found that two doses of the Pfizer vaccine offered 70 per cent protection against severe hospitalisation and death, down from about 95 per cent before Omicron was detected.

At Wednesday's WHO meeting, one scientist after another presented similar laboratory findings showing that vaccine-induced antibodies performed much worse against Omicron than against other variants.

But boosters seem to provide enough extra antibodies to lessen these infections. Dr Fauci described experiments at the National Institutes of Health, in which scientists took blood serum from people who had two doses of the Moderna vaccine as well as from others who had a third dose. The researchers then mixed the serum with viruses engineered to carry Omicron's surface proteins.

These "pseudoviruses" evaded many antibodies from people who had received two doses of Moderna, but the boosters produced such high levels of antibodies that the viruses were blocked from invading cells.

"So the message remains clear: If you are unvaccinated get vaccinated, and particularly in the arena of Omicron, if you are fully vaccinated, get your booster shot," Fauci said.

Dr Fauci's admonition comes as Biden administration officials are bracing for a potential wave of Omicron infections that could overwhelm the health care system.

The Centres for Disease Control and Protection warned recently that the percentage of coronavirus cases in the United States caused by the Omicron variant had increased sharply and might portend a significant surge in infections as soon as next month. The Delta variant remains by far the dominant version across the United States.

In anticipation of that wave, the administration is trying to encourage all Americans who may be eligible - those 16 and older who received their second vaccine dose at least six months ago - to get their booster shots. About 27 per cent of fully vaccinated Americans have also had booster shots, according to the CDC.

Many countries are rushing boosters to their populations, but Omicron is spreading so fast it may well outstrip even the best efforts.

"The projected transmission rates, if borne out, do not give us much time for interventions," Dr Phil Krause, a former vaccine regulator at the Food and Drug Administration, said at the WHO meeting.

That prospect has led many scientists to hope that T cells will serve as an effective backup when antibodies fail. If these immune cells can fight Omicron, they may prevent many infections from turning into severe disease.

After a cell is infected with the coronavirus, T cells can learn to recognise fragments of viral proteins that end up on the cell's outer surface. The T cells then kill the infected cell, or alert the immune system to launch a stronger attack against the virus.

Dr Alessandro Sette, an immunologist at the La Jolla Institute for Immunology, and Dr Andrew Redd of the National Institutes of Health reported that despite Omicron's many mutations, most of the protein fragments recognised by T cells are identical to those of other variants.

Those findings suggest that T cells trained by vaccines or previous infections will respond aggressively to Omicron, rather than standing by. "It appears the T cell response is largely preserved," Dr Sette said.

Dr Burgers and her colleagues tested that possibility by collecting T cells from 16 people vaccinated with two doses of the Pfizer vaccine and exposing those T cells to protein fragments from the Omicron variant. The scientists found that the response of the T cells to the variant was about 70 per cent as powerful as their response to the original form of the virus.

A number of scientists at the meeting cautioned that these data come from studying cells in a laboratory, known as in vitro experiments. It will take a few more weeks of examining infections in people before it becomes clear how well T cells prevent severe disease.

"We don't know yet what these in vitro findings actually mean for disease severity," said Dr Nora Gerhards, a virologist at Wageningen University in the Netherlands. "And that's what it's all about. Because in the end we want to prevent a collapse of the health care systems in our countries."