When the Pfizer and Moderna COVID-19 vaccines first became available in December 2020, there was a strict dosing schedule recommended for the inoculations.
People who got the Pfizer shot were advised to wait 21 days before getting dose two, and those who got Moderna waited 28 days. For the most part, people in the United States adhered to this schedule, but in areas where supply was tight, people had to wait a few extra weeks before they could access a second dose.
In the United Kingdom and Canada, for example, limited supply meant that people often had to wait anywhere from eight to 12 weeks for their second dose. Many individuals in this situation were understandably concerned about how delaying the second dose would ultimately impact their protection — but data would eventually show that extending the waiting period for the second dose was not a problem, and was actually beneficial.
“There’s a lot of evidence now that spacing out the vaccines was a good idea to begin with,” Monica Gandhi, an infectious diseases specialist with the University of California, San Francisco, told HuffPost.
Here’s what we’ve learned about spacing out COVID-19 vaccine doses.
Studies and real-world evidence have shown that extending the time between the first and second doses of the Pfizer and Moderna shots improves immunity.
A paper published in the journal Nature found that extending the duration between the Pfizer doses to 12 weeks led to a more robust antibody response. Another recent study from the Mayo Clinic found that people who got vaccinated with Pfizer were more likely to get a breakthrough infection compared to those who got the Moderna shots.
“Even four weeks made a difference as opposed to having three weeks between doses,” Gandhi said.
The real-world evidence from Canada and the U.K., where vaccine scarcity forced people to separate their doses up to four months, compared to the United States and Israel ― where there were plenty of vaccines ― also suggests there’s some added protection in delaying the second dose.
The vaccine efficacy against symptomatic illness “is lower in Israel and the U.S. and both countries opted for a three-week strategy for Pfizer, whereas Canada and the U.K. both have not reported reduction in vaccine effectiveness against symptomatic COVID,” Gandhi said. (Note: both vaccines, regardless of when the doses were given, remain over 90% effective against hospitalizations and death, which is what really matters.)
We often space out doses with other shots.
Scientists who study vaccines weren’t surprised to learn that extending the window between doses produced a better immune response.
“This has been known long before COVID-19,” said Isaac Bogoch, an infectious diseases physician and scientist with University of Toronto.
Many of the vaccines used to protect us against other infectious diseases have multiple doses that are separated by months. Why? Because data from vaccinology, in general, suggests that extending time between doses strengthens immunity.
A 2018 study on messenger RNA (mRNA) vaccines from before the pandemic found that when mRNA doses are spaced out longer, the body creates stronger T cell and B cell responses (the other arm of the immune system that works alongside our antibodies).
This idea of extending the time between doses is not an accident, Bogoch said. “People will form a more robust immune response with the doses separated apart,” he explained.
The two Hepatitis A shots are separated by six months. With the Hepatitis B shot, the second dose is given four weeks after the first, and the final dose is given eight weeks after the second. Children getting the measles-mumps-rubella vaccine routinely get the second dose three months after the first — but it’s OK if they miss the exact time and come in a couple months later.
You’re still well protected either way.
This doesn’t mean you should be concerned if you got your second Pfizer dose 21 days after your first or the second Moderna shot 28 days later. You’re definitely still protected — the vaccine clinical trials that were designed at these intervals show great vaccine effectiveness against severe disease.
“It kind of can give you a sense that no matter how you do it, the vaccines work well,” Gandhi said.
Bogoch added it’s not unreasonable, given the circumstances, that the doses for the COVID-19 vaccines were separated by a narrow 21 to 28 days. If we had separated doses by several months, it would have taken much longer to conduct the clinical trials and then vaccinate people and build up population immunity all in the midst a global public health crisis.
Looking forward, Bogoch said it’ll be fascinating to watch what happens in populations that had their doses delayed versus populations that didn’t. That data will help inform future vaccine strategies and help us determine if and when boosters should be given.
“To what extent does separating these doses provide greater protection ― not just to infection but also to severity?” Bogoch asked. “The cool thing is this is an answerable question. This is going to be done.”
Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.
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