A UK study has found that some patients with impaired immune systems have fewer or no antibodies after receiving two doses of a COVID-19 vaccine.

Though experts do not know what effect this will have on protection against the virus, they say booster shots may be a good idea for some immunosuppressed people.

The Octave Study saw 600 patients with cancer, inflammatory arthritis, kidney disease, liver disease, and recipients of stem cell transplants have their blood tested to detect their levels of antibodies after being vaccinated.

The findings from the study, published as a pre-print in The Lancet, suggest that 40 percent of participants had a sub-optimal antibody response after two doses of AstraZeneca or Pfizer.

Additionally, 11 percent of those with a sub-optimal response had no detectable antibodies four weeks after receiving the second dose.

Many of these patients were taking a strong medicine called rituximab, used to treat vasculitis.

Promisingly, the remaining 60 percent of volunteers had similar antibody levels to young people, as well as optimal levels of T-cells – another type of immune cell that can destroy coronavirus cells.

“While 40 percent of these clinically at-risk patient groups were found to have a low or undetectable immune response after a double dose of the vaccine, we are encouraged that this figure isn’t higher,” said Professor Iain McInnes, the lead researcher of the Octave trial.

“However, it is possible even partial protection may be clinically beneficial, and this is something we will closely monitor.”

Professor Eleanor Riley, an expert in immunology and infectious diseases at the University of Edinburgh agreed.

“As it is T-cells that are particularly effective at stopping us getting severely ill and needing hospital treatment, we would expect that the vaccine is still offering substantial protection to most of those highly vulnerable people,” she said.

Professor Charles Swanton from Cancer Research UK said: “We know the results could be worrying for those who are clinically vulnerable, but anyone undergoing cancer treatment should continue to follow the advice of their doctors and we encourage all who can to get the vaccine.”

This article first appeared on Over60.