Antibodies produced by immune cells become steadily more formidable and precisely targeted against the SARS-CoV-2 virus for at least six months after COVID-19 vaccination, according to a study.

The researchers at Washington University School of Medicine in the US assessed the antibody response to the Pfizer-BioNTech vaccine in detail in people. The findings, published on February 15 in the journal Nature, suggest that declining antibody levels in the months after vaccination primarily represent a shift to a sustainable immune response. The researchers also noted that even quite low levels of antibodies would continue to provide some protection against disease, as long as the virus does not change.

"If the virus did not change, most people who got two doses of this vaccine would be in very good shape," said senior study author Ali Ellebedy, an associate professor at Washington University. "The antibody response we saw is exactly what we would expect from a robust immune response. We never thought that six months following that second injection, many people would still be actively improving the quality of their antibodies, Ellebedy said. The researchers collected blood from 42 participants and lymph node samples from 15 participants before each person received their first dose of the Pfizer COVID-19 vaccine and at weeks three, four, five, seven, 15 and 29 afterwards.

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

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. It indicates a strong antibody response that continued to mature and improve, he said. Six months after vaccination, the antibodies were noticeably better than they had been in the beginning, they said. In one set of experiments, the researchers found that only 20 per cent of early antibodies are 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, they said.

"When you look at antibodies, quantity should not be your only concern. The antibodies at six months might be less in quantity, but they are much better in quality," Ellebedy said. The researchers, however, noted that the quality of the antibodies is measured against the original virus that was used to design the vaccine. If a new variant is different enough from the original, it may be able to escape once-powerful antibodies, they added.