Is It Okay To Delay Your Second Vaccine Shot?

Have you ever been so incredibly hyped about an event — a first date or an interview — that you showed up ridiculously early? Well, that’s exactly how I handled getting my Pfizer/BioNTech vaccine earlier this month. I was overflowing with excitement about it, like a flute with fizzy Champagne bubbling over the rim. But those effervescent feelings were so strong, that they caused me to only skim the details of my appointment — and show up way too early at a CVS in Staten Island. Twenty-four hours early, to be exact. 

It was a major screw-up on my end, but luckily the nice people at the pharmacy squeezed me in anyway, since I’d traveled an hour and a half by train and bus to get there. However, that day at the pharmacy, I was told to just keep my original appointment for my second shot of the mRNA vaccine. At first, I worried about the timing. Because of my mix-up, I’d be getting my other dose 22 days after the first, instead of 21. Could that be a problem? 

The Centers for Disease Control and Prevention suggests specific intervals between shots for the mRNA vaccines, based on trial data. During the Pfizer trial, shots were administered 21 days apart. During the Moderna study, they were given 28 days apart. So, those are the intervals that are proven to work. But luckily for people like me, there’s a fairly generous grace period, and if you get your shot within it, you’ll still be reaping the benefits of immunization. 

“Second doses administered within a grace period of four days earlier than the recommended date for the second dose are still considered valid,” the CDC notes. They add that it’s cool to get your second dose of either vaccine up to six weeks, or 42 days, after the first dose “if it is not feasible to adhere to the recommended interval.” All this is moot for the Johnson & Johnson vaccine, which requires just one shot.

“It’s best to have the intention to get your second shot of an mRNA vaccine after the recommended number of days, but there may be instances where people have to wait or get it a little early” says Anurag Malani, MD, medical director of Infection Prevention & Antimicrobial Stewardship Programs at St. Joseph Mercy Health System. “Life happens, and not everyone is able to come exactly three or four weeks after their first dose. They could have a rare occurrence of developing COVID after the first dose, and have to quarantine. Or maybe they miss the appointment because of work. They can still get it within the grace period and the vaccine will be safe and effective for them.” After two doses, the Pfizer vaccine is 95% effective, and Moderna is 94.1% effective at preventing COVID-19 in those without prior infection, and that immunity lasts for around six months at least.

Different countries have taken differing approaches to distributing vaccines, with some knowingly delaying second shots. In Britain, for example, the National Health Service decided to delay administering second shots until up to 12 weeks after the first. The decision was based on evidence suggesting that spacing out the doses may improve effectiveness in some cases. 

Others say, however, that there’s not enough data to support this approach, and it has a major downside: People aren’t fully protected during the three month interval between shots. “We have to stick with what we know works,” Nancy Messonnier, the director of the Centers for Disease Control and Prevention’s National Center for Immunization and Respiratory Diseases, told STAT News. “This is the regimen that’s been carefully studied. We’ve promised the American public that we would follow the data and follow the science, and that’s what we’re doing.”

As such, Dr. Malani emphasizes that it’s best to shoot for the CDC recommended interval between doses, or as close to it as possible. Even outside of the 42-day grace period, though, Dr. Malani says that there’s still a benefit to getting the second dose; just schedule it as soon as you can. Although scientists don’t know for sure how efficacious the second shot will be when it’s given more than six weeks after the first, it’s reasonable to assume, based on the data, that it will still be effective — more so than just getting one shot and skipping the second entirely. 

The second shot is critical for the mRNA vaccines because it acts as a booster. While studies have indicated that the Pfizer and Moderna vaccines are between 52% and 80% effective after one dose, so far experts don’t know how long that immunity will last. You may be 80% less likely to contract COVID-19 in the three weeks post-shot, but the longer you go without getting a booster, the more your immunity could wane. 

“The bottom line is your immunity will last longer and be more robust with a second dose,” Dr. Malani says. “And it’s best to have the intention of getting that dose on schedule.” 

So I don’t have to worry about waiting 22 days to get my second dose, compared to the recommended 21 days. But anyone who’s thinking of skipping the second shot altogether should think again.