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Does the Interval Between COVID-19 Vaccine Doses Matter?

Does the Interval Between COVID-19 Vaccine Doses Matter?
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Published on April 1, 2021

Research Findings on COVID-19 Vaccine Doses and Timing

Patients with cancer who received only one dose of the Pfizer vaccine were less protected against COVID-19 than individuals without cancer who received a single dose of the vaccine, according to a study from King’s College London.

The study was published as a preprint, which means it has not been evaluated and should not be used to guide clinical practice — however, the findings suggest that patients with cancer should make sure to receive their second dose of the Pfizer vaccine within the recommended 21-day window to provide the best chance that they have some protection.

Researchers monitored 151 mostly elderly patients with solid and blood cancers and compared results from those who received their second dose within three weeks and those who did not. After three weeks, the effectiveness of a single dose in patients with solid cancers, including lung cancer, breast cancer and bowel cancer, was 39%. It was even lower in patients with blood cancers (13%). It was 97% effective in the 54 participants without cancer.

In patients with solid cancers, the efficacy was greatly and rapidly increased by a second dose within three weeks (95% within two weeks of the second dose). Too few patients with blood cancers were boosted for clear conclusions to be drawn, the authors said. Among those patients who had to wait longer for their second dose, there was no real improvement in protection.

“Delayed boosting potentially leaves most solid and hematological cancer patients wholly or partially unprotected, with implications for their own health; their environment and the evolution of VOC (variant of concern) strains,” the authors concluded. “Prompt boosting of solid cancer patients quickly overcomes the poor efficacy of the primary inoculum (first dose) in solid cancer patients.”

Why Two Doses of the COVID-19 Vaccine?

In clinical trials for the Pfizer vaccine, one dose was given at Day-1 followed by a Day-21 booster. This resulted in a 95% efficacy at preventing COVID-19, including severe disease, the researchers note. (The efficacy between the first and second dose was 52%, though a separate analysis found that the efficacy was closer to 89% or 91% 15 days after the first dose.)

However, of the 18,860 individuals immunized with the vaccine, none of them had cancer, the study notes. Those with a history of COVID-19, treatment with immunosuppressive therapy or an immunocompromising condition were excluded.

The Centers for Disease Control and Prevention (CDC) recommends that the second dose of the COVID-19 vaccine be given within three weeks of the first dose for the Pfizer vaccine and no more than six weeks after. If the second dose is not given during these time frames, it can be given without the need to repeat the vaccine series, the government agency reports.

It is not known what will happen if people never get a second dose, according to an article published in JAMA. It is possible that people who receive a single dose will only develop partial immunity, resulting in a higher risk that vaccine-resistant variants will develop, wrote Dr. Edward Livingston, who has since resigned as deputy editor of the journal. 

“There is also concern that people who get only one dose will think they have sufficient protection against COVID-19 and not get a second dose,” he wrote. He added that there is no evidence that people who get only one dose have adequate long-term protection against COVID-19 infection.

I Can’t Get My Second Dose of the COVID-19 Vaccine, Should I Be Concerned?

In the United States, both the Pfizer and Moderna vaccines are being administered as they were intended to — with second doses given 21 and 28 days after first doses, respectively.

However, if for some reason you are unable to get the second dose (you miss the appointment, there is a long line and you can’t afford to wait, there is bad weather, supply shortages, etc.), try to reschedule it as close to the original date as possible. Contact the location that set up your first appointment for help.

Whatever you do, don’t give up trying to get the second dose. While one shot may give you some protection, you need the second dose for full immunity.

Still, even with two doses of the COVID-19 vaccine, patients with cancer may not “develop a full immune response due to their cancer or cancer treatment,” the Leukemia & Lymphoma Society states on its website. “That’s why it’s important for cancer patients to continue to take other preventative measures like hand washing, wearing masks and social distancing, even after vaccination.”

On March 5, Patient Power hosted a Facebook Live discussion on the COVID-19 vaccine. Watch the replay for more information.

~Megan Trusdell 

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