Nearly 100% of patients with solid tumors have antibodies effective against the SARS–CoV-2 delta variant after a third dose of COVID-19 vaccine, according to results published as a correspondence by Fendler et al in Cancer Cell. The new findings also highlight a proportion of patients with blood cancers who still have undetectable antibody levels against the delta variant after three doses.

As part of the ongoing CAPTURE study led by the Francis Crick Institute and The Royal Marsden NHS Foundation Trust, researchers have been monitoring the immune responses of hundreds of patients with different types of cancer after one, two, and three doses of COVID-19 vaccine.

Measuring Antibody Levels

Using a highly accurate test—a viral neutralization assay—the research team measured levels of antibodies which specifically block the delta variant from infecting cells. In 199 people with cancer who had received a third vaccine dose (115 with solid tumors and 84 with blood cancers), they assessed whether levels of these neutralizing antibodies in the blood were sufficient to block at least 50% of virus infection under laboratory conditions.

To examine the added benefit of a booster, the team specifically analyzed responses in patients who had not shown a neutralizing antibody response against delta after their second vaccine dose, or in patients whose response had waned since. They found that after a third dose, 94% (47 of 50) of patients with solid cancers had newly detectable levels of neutralizing antibodies against delta. While a third vaccine dose also effectively boosted antibody levels for many patients with blood cancers (54%, or 28 of 52), a substantial proportion still had undetectable levels in their blood.

Overall, across all patients studied after three doses, 97% of patients with solid cancers and 71% of patients with blood cancers had detectable antibody levels against delta.

In a previous analysis of patient responses after two vaccine doses published by Fendler et al in Nature Cancer, the researchers had found that 31% of patients with blood cancer and 62% of patients with solid cancers had detectable antibody levels against delta. The team suggested that low vaccine protection and a reduction in protective measures likely contributed to people with blood cancer now accounting for a higher proportion of COVID-19 deaths.

Samra Turajlic, MBBS, PhD, lead study author and group leader at the Crick and Consultant Medical Oncologist at The Royal Marsden NHS Foundation Trust, said, “Overall, we’re seeing the positive effects of vaccination in patients with cancer, who we know are more vulnerable to COVID-19 infection. Even for people with blood cancers, we’re seeing some who initially weren’t responding to the vaccine start to develop antibodies after three doses.”

The team also examined differences in the types of vaccines patients had received. In patients with blood cancers, they found that a third dose of the Pfizer-BioNTech vaccine was more likely to boost antibodies to detectable levels if the patient had the Oxford-AstraZeneca vaccine for their first and second dose.

Importantly, the researchers were also able to study T-cell responses in a subset of the patients, helping to fill a significant gap in understanding of the wider immune response to COVID-19. Overall, they found that a third dose also effectively boosts T-cell levels in patients with both solid and blood cancers.

“As we face a new, more infectious variant in Omicron, it’s increasingly important to prioritize protection for vulnerable patients,” added Dr. Turajlic. “Our team will continue to examine changes in immune response as the pandemic evolves.”

Disclosure: For full disclosures of the study authors, visit cell.com.