New York, Feb 16 (IANS): For at least six months after Covid-19 vaccination, antibodies produced by immune cells become steadily more formidable and more precisely targeted against the virus that causes Covid-19, according to a study of the antibody response to the Pfizer-BioNTech vaccine.

The findings, published in the journal Nature, suggest that declining antibody levels in the months after vaccination primarily represent a shift to a sustainable immune response.

Producing vast quantities of antibodies burns a lot of energy. The immune system cannot sustain such a high level of activity indefinitely, so it gradually switches to producing smaller amounts of more powerful antibodies.

Even quite low levels of antibodies would continue to provide some protection against disease, the researchers said -- as long as the virus doesn't change.

"If the virus didn't change, most people who got two doses of this vaccine would be in very good shape," said researcher Ali Ellebedy from the Washington University School of Medicine in St. Louis.

For the study, the team collected blood from 42 participants and lymph node samples from 15 participants before each person received his or her first dose of the Pfizer-BioNTech Covid-19 vaccine and at weeks three, four, five, seven, 15 and 29 afterward.

The researchers also obtained bone marrow samples from 11 participants 29 and 40 weeks after the first vaccine dose.

Eight people provided all three kinds of samples, allowing the researchers to track the development of the antibody response over time within those individuals. None of the eight had been infected with the virus that causes Covid-19, so their antibody responses were entirely due to vaccination.

The researchers found that B cells targeted against SARS-CoV-2 persisted in the germinal centres - like boot camps where B cells are trained to make ever-better-quality antibodies - of all participants for months. Even six months after vaccination, 10 out of 15 people still had B cells in their germinal centres.

The more time B cells spend in germinal centres, the more potent their antibodies get. Germinal centres had been thought to last only a few weeks, so finding these boot camps still training B cells in a majority of people so long after vaccination was a surprise, Ellebedy said, and an indication of a strong antibody response that continued to mature and improve.

Indeed, six months after vaccination, the antibodies were noticeably better than they had been in the beginning. In one set of experiments, the researchers found that only 20 per cent of early antibodies bound to a protein from the virus. Six months later, nearly 80 per cent of antibodies from the same individuals bound to the viral protein.