TORONTO -- Experts are ringing the alarm bell over the prevailing policy of delaying the second dose of the vaccine for up to four months, saying this strategy could leave cancer patients with less protection against the virus.

Citing preliminary research from the U.K., the Canadian Association of Pharmacy in Oncology (CAPhO) said in a statement Wednesday that cancer patients are significantly less protected by a single dose of the Pfizer-BioNTech vaccine than the general public.

“The study shows that three weeks after one dose of the vaccine an [antibody] response was found in 39 per cent of people with a solid cancer and just 13 per cent of people with blood cancer,” Tina Crosbie, President of CAPhO, said in the statement. “An antibody response was found in 97 per cent of the healthy volunteers tested.”

Cancer patients and other immunocompromised people are significantly more likely to experience severe outcomes or die if they contract COVID-19, making it more critical that they receive protection from the vaccine.

CAPhO is calling for the National Advisory Committee on Immunization (NACI) to specify in their recommendations that cancer patients receive their second dose on schedule, and no later than three weeks after the first dose.

In order to get as many people their first shot as possible, NACI recommended in early March that the second dose be pushed back up to four months.

However, NACI revealed on Wednesday that recent data regarding the pace of the vaccine rollout indicates that the interval between the first and second dose will not be four months in most cases. 

Most Canadians will likely receive their second dose within 1.5 to two months after the first, NACI said.

Responding to this news, the CAPhO said in a second statement that they join Canadians “in being relieved in the increased availability of vaccines arriving to our country.”

Crosbie told in an email that it is “a step in the right direction.”

However, CAPhO reiterated that cancer patients should be earmarked as a group in particular need of receiving their second dose of the vaccine on time.

“Increasing supply to our country is great news but delaying the second dose is still a delay of immune response for these vulnerable Canadians,” CAPhO stated.

“NACI and public health officials have been invaluable in identifying high risk groups throughout the vaccine rollout. CAPhO encourages NACI to include patients with cancer as another vulnerable population who require the second dose within three weeks.”

There is not a lot of data available on how vaccines perform in immunocompromised patients such as cancer patients, as large vaccine trials did not include these populations. While experts emphasize that the vaccines are safe for cancer patients to receive, it’s unclear whether the efficacy of the vaccines is as high in cancer patients as it is in the general population.

This was the question the U.K. study referenced by CAPhO aimed to investigate. This research is still in preprint, which means it has not yet been peer-reviewed.

Researchers studied 151 cancer patients at three hospitals in the U.K. and 54 healthy control patients, most of whom were health-care workers, between Dec. 8, 2020 and Feb. 18.

They took blood samples of the participants pre-vaccination, three weeks after the first shot of the vaccine and also five weeks after the first shot of the vaccine.

Only 31 of the cancer patients received their second dose of the vaccine 21 days after the first, while 118 were scheduled to receive their second dose in 12 weeks' time. Out of the non-cancer patients, 16 received their second dose on time, while 38 had a second dose date of 12 weeks later.

Non-cancer patients had a markedly higher percentage of exhibiting an immune response three weeks after the first shot of the vaccine than cancer patients, but that wasn’t the end of the differences.

The timing of the second dose had a significant impact on the percentage of cancer patients who mounted an antibody response.

The efficacy of the vaccine in solid cancer patients — patients with a physical tumour in one of their organs — at the five-week mark was 95 per cent in those who received their second dose 21 days after their first dose. These results are in line with the Pfizer phase 3 trials in the general population.

However, when solid cancer patients were assessed at the five week mark, only 43 per cent of those who had not received their second dose yet still showed an immune response in the form of antibodies.

Only eight per cent of those with a blood cancer and just one shot of the vaccine showed an immune response at five weeks.

Although 71 per cent of solid cancer patients showed a T-cell response — another aspect of the immune response — 21 days after their first shot, not far behind the 82 per cent of non-cancer patients who were assessed, the difference in antibody production if the second shot is delayed too long leaves experts concerned.

“We recognize that the SOAP-02 UK study is emerging data,” CAPhO said in their second statement. “However, it has been identified elsewhere that cancer treatments can reduce the effectiveness of vaccines. To this end, patients with cancer need the second dose within three weeks to mount an immune response.”