(CNN) — The Pfizer/BioNTech Covid-19 vaccine provides less protection in cancer patients than healthy individuals following a single dose, a new real-world study in the UK suggests, raising questions about whether the UK’s strategy to delay second doses should apply to such patients.

A second dose of the vaccine at three weeks, however, boosted their protection significantly, with the researchers calling for earlier boosts in this group in the UK. The UK’s vaccine strategy currently involves a 12-week gap between doses of the coronavirus vaccines; Pfizer recommends 21 days between doses.

The study analyzed the impact of the Pfizer-BioNTech vaccine on 205 participants — 54 healthy volunteers and 151 elderly patients with solid cancers, such as breast or prostate cancer, and haematological (blood) cancers, such as leukemia. The preprint study has not yet been peer-reviewed or published.

The researchers looked for levels of antibodies and T cells in their blood to identify the level of immune response generated against the coronavirus.

Three weeks after one dose of the vaccine, an antibody response was found in 39% of solid cancer patients and just 13% of people with blood cancer. The response in healthy volunteers was 97%.

In the solid cancer patients who received a second dose three weeks after the first, the antibody response shot up to 95% within two weeks of the boost. There were not enough booster vaccines given to blood cancer patients to determine the response in that group.

Further evidence of the need for a boost was shown by the fact that antibody levels only increased to 43% in people with solid cancers and 8% in those with blood cancer five weeks after their first dose. It was 100% in healthy volunteers.

“Our data provides the first real-world evidence of immune efficacy following one dose of the Pfizer vaccine in immunocompromised patient populations. We show that following first dose, most solid and haematological cancer patients remained immunologically unprotected up until at least five weeks following primary injection; but this poor one dose efficacy can be rescued with an early booster at day 21,” said Dr. Sheeba Irshad, a senior clinical lecturer from the School of Cancer & Pharmaceutical Sciences who led the research.

“Based on our findings, we would recommend an urgent review of the vaccine strategy for clinically extremely vulnerable groups. Until then, it is important that cancer patients continue to observe all public health measures in place such as social distancing and shielding when attending hospitals, even after vaccination,” Irshad added in a statement.

Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, highlighted that certain limitations needs to be taken into account.

“The change in UK policy around delaying a second dose of vaccine allowed for the authors to make some comparisons between those who received a second dose within 21 days and those who did not. They have not yet provided data on those who received a second dose after a 12 week delay,” he said in a statement to the UK’s Science Media Centre.

“Nevertheless, these results do suggest that the vaccines may well not protect those patients with cancer as well as those without cancer,” he added.

“All of these findings are consistent with our understanding of the immune system function in cancer patients.,” said Shoba Amarnath, Newcastle University research fellow at the Newcastle University Centre for Cancer. “We know that the immune system within cancer patients is compromised as compared to healthy controls. Hence, a 2nd vaccine boost prepares the dysregulated immune system to function at the same efficiency as healthy controls.

“The data in the study supports the notion that in solid cancer patients a considerable delay in second dose will extend the period when cancer patients are at risk of SARS-CoV-2 infection.”

The-CNN-Wire

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