Not everyone receives the same level of protection from their COVID shots, even those who got the same vaccine. Studies have shown that anything from your age to your underlying health conditions could lower your immune response. And with breakthrough infections seemingly occurring more often as the Delta variant spreads through the U.S., vaccinated people are becoming more concerned about how strong their immune response to the vaccine was. Fortunately, new research has found that certain factors may boost your protection after vaccination—even against the Delta variant. Now, a new study out of the U.K. has determined that some Pfizer recipients may be more protected against the Delta variant, depending on how they spaced out their doses.
The study—which was released as a preprint on July 23, but has not yet been peer reviewed—looked at the immune response among 503 U.K. healthcare workers who received the Pfizer vaccine between Dec. 9 and May 23. Researchers from various U.K. universities tested and analyzed the antibody and T cell responses (which are used to determine immunity levels) among the workers against different COVID variants. Then, they compared results between those who had a short interval in between doses (two to five weeks) and those who had a long interval (six to 14 weeks).
In the U.K., first doses were administered to everyone eligible first before second doses were rolled out, which is why the length of time varies so wildly. In the U.S., on the other hand, a 21-day interval—or as close to it as possible—was the standard for all Pfizer recipients.
According to the study, both the antibody and T cell responses against the Delta variant were higher in Pfizer recipients who waited longer in between doses. The researchers found that delaying the second dose up to 10 weeks boosted antibodies and T cells higher than the recommended three-week interval.
The "sweet spot" for the most protection against the Delta variant was eight weeks between Pfizer doses, Susanna Dunachie, PhD, a National Institute for Health Research global research professor at Oxford University who co-led the study, said during a July 22 news briefing, per Business Insider.
But, no matter how many weeks you waited between doses, the researchers noted that two Pfizer shots produced a significantly higher immune response than just one dose. The challenge is that waiting longer to get the second dose does also pose a risk because then the recipient is not protected as well against the virus in the weeks after their first shot, before before their final one. For some groups of people, waiting longer in between doses is even more risky. Lance Turtle, PhD, one of the study's researchers and a senior clinical lecturer in infectious diseases at the University of Liverpool, said that immunosuppressed individuals, for example, should be getting their second dose as soon as possible.
Though they haven't responded directly to this study, the Centers for Disease Control and Prevention (CDC) has maintained that the second Pfizer shot should be administered 21 days after the first, saying that people should get their second shot "as close to the recommended three-week interval as possible."
However, the agency says that if necessary, your second dose can be given up to six weeks after the first. And more recently, CDC Director Rochelle Walensky, MD, said that no matter how much time has passed, you should get the second dose if you're getting vaccinated with one of the two mRNA vaccines, Pfizer or Moderna.
"Pfizer and Moderna vaccines are most effective, especially against the Delta variant, when given as two shots in a series," Walensky said during a July 16 White House COVID briefing. "We encourage that people get vaccinated on schedule three or four weeks after your first dose. But if you are beyond that window, I want to reiterate: There is no bad time to get your second shot."