- Giving two doses of Pfizer’s shot eight weeks apart could best protect against the Delta variant.
- A study found that the eight-week delay boosted immune responses higher than waiting 21 days.
- It’s a trade-off between needing two doses and optimizing the immune response, the researchers said.
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A study posted Friday, led by Oxford University, found that delaying the second dose up to 10 weeks boosted antibodies and “helper” T cells that support the immune system higher than giving it at three weeks, as recommended by Pfizer.
Two doses of Pfizer’s shot boosted immune responses higher than one dose, no matter the time between doses, the study authors from the universities of Oxford, Newcastle, Liverpool, Sheffield, and Birmingham added. Waiting longer to get a second dose presents a trade-off because people are less protected against the coronavirus after just one shot.
A ‘sweet spot’ for protection from the coronavirus
Susanna Dunachie, a National Institute for Health Research global research professor at Oxford University who co-led the study, said at a news briefing on Thursday that eight weeks between doses for Pfizer’s vaccine “was the sweet spot.”
The study authors cautioned that “regardless of the dosing schedule, the study found levels of antibodies and T cells varied significantly from person to person, which may depend on genetics, underlying health conditions, and past exposure to COVID-19 and other viruses.”
The Protective Immunity from T cells to COVID-19 in Health workers, or PITCH, study, which hasn’t yet been scrutinized by other experts in a peer review, used blood samples from 503 health workers who were mostly white (86%) and women (74%), with an average age of 43.
Dr. Lance Turtle, senior clinical lecturer in infectious diseases at University of Liverpool, was also involved in the study. He said that eight weeks between doses is a “reasonable compromise” but added that there are exceptions. People who are immunosuppressed, such as those being treated for cancer, should get second doses of Pfizer’s shot as soon as possible, he said.
The UK delays the second dose by up to eight weeks so that more people can get their first shot and because a longer interval between doses worked better for other vaccines.
There are risks to delaying the second shot
In the US, the two doses of Pfizer’s vaccine are given at 21 days apart, as recommended by Pfizer. Dr. Anthony Fauci, President Joe Biden’s chief medical advisor, has previously said that delaying second doses could produce a weaker immune response that nurtures variants because people aren’t fully protected until after they get both doses.
Nadhim Zahawi, UK vaccines minister said in a press release that the study findings were “hugely significant” because they help us “better understand the mechanics behind our immune response to COVID-19 and the importance of getting both doses of the vaccine.”
Previous real-world studies from the UK have shown that Pfizer’s vaccine was 88% effective at preventing symptomatic COVID-19 caused by Delta after two doses but just 33% effective after a single shot. But the optimum time between doses to protect against variants is not clear. We also don’t know whether delaying doses affects how long protection lasts.
Dr. Rebecca Payne, an immunologist at Newcastle University who was involved in the study, said that the PITCH study provided “reassuring evidence” that both dosing schedules generate robust immune responses against COVID-19 after two doses.
“We now need to carry out more follow-up studies to understand the full clinical significance of our findings,” she said.