To date, more than 142 million adults in the United States have received at least one dose of the vaccine. Some people, however, are stopping at that single dose. In fact, more than 5 million people have skipped their second dose of the Pfizer BioNTech or Moderna vaccines, which amounts to 8 percent of the total number of people who got their first dose.

Yes, the second shot can result in unpleasant side effects, such as fever, chills, and body aches. And that might make it tempting to skip your second dose, but public health officials are begging people to follow through. If you skip the second dose, you’re putting yourself at others at unnecessary risk.

Even if you had an unpleasant experience after the first shot, there are a few things an epidemiologist wants you to know before you even think about skipping the second.

Why it’s so important that you don’t skip your second dose of the COVID-19 vaccine

1. You may only be 40 percent protected after your first shot

The primary reason to get your second dose of the vaccine is that you’re not as protected from contracting or transmitting COVID-19 without it. According to Timothy Brewer, MD, professor of medicine and epidemiology at UCLA, data shows that the vaccine’s efficacy is “substantially higher” after the second dose when compared to the first dose. “For example, if you look at the mRNA vaccines—the Pfizer and the Moderna—the efficacy after one dose ranges somewhere between 40 and 80 percent, so the protective effect of the vaccine is 40 to 80 percent, depending on how far out you are from when you got the dose,” he says. “If you get the second dose of either of those two vaccines the protective effect goes up to 95 percent—so you do get a substantial boost by getting that second dose.”

While the difference between 80 percent efficacy and 95 percent efficacy may not seem all that significant, Dr. Brewer says it’s important to remember that these numbers are averages, which means you could be on the lower end of the spectrum and therefore be potentially far less protected than 80 percent after the first shot.

The side effects aren’t all that different between the first and the second dose, but the protection effects are clearly worth it. “There’s really not much risk to the second dose, and there is a lot of benefit,” he says. “If you do what we all a risk-benefit analysis, you’re giving up a fair amount of benefit with no real reduction to your risk, so that doesn’t make intuitive sense.” And while you may have heard others report varying degrees of more severe sickness the day after their second dose than they experienced after their first dose, there is not much risk from those symptoms aside from temporary discomfort.

2. If you only got the fist dose, you may still transmit the disease to someone else and help enable its mutation

The higher the protective effects of your vaccine regimen, the more likely you are to be protected from serious disease, hospitalization, and death. But you’re also more likely not to transmit it to someone else, says Dr. Brewer. Obviously, this ensures fewer, if any, people get sick as a result of contact with you—which is a good thing.

Preventing this spread helps stop the virus from mutating into new strains against which current vaccines may be less effective, too. “If you can catch COVID-19 because you’re not fully-vaccinated, for example, you can transmit it, and when we have high levels of circulating virus it is that much easier for it to mutate,” says epidemiologist Beth Linas, PhD.

3. Fully-vaccinated people have more fun

The third reason to get fully-vaccinated via a second dose, says Dr. Brewer, is that the Centers for Disease Control and Prevention (CDC) just released new guidelines as to what fully-vaccinated people can do relative to people who have not been fully vaccinated. “You can now go outdoors without a mask if you’ve been fully vaccinated, you can travel domestically without having to be tested or self-quarantined, you don’t have to quarantine after unknown exposure if you’re asymptomatic, you don’t have to participate in routine screenings if you’re asymptomatic,” says Dr. Brewer. “There are a number of things that fully-vaccinated people can do that not vaccinated or partially-vaccinated people should not do, and that’s another reason to go out and get that second dose.”

How to get your second shot if you missed your appointment or your provider ran out of supply

If you skipped your first dose and are now having second thoughts (as you should!) or you weren’t able to get your second dose due to supply snafus, Dr. Brewer suggests you contact your primary care provider, try accessing information through your county’s public health department, or go to the CDC’s vaccine locator. “Even if you missed the window [and it’s past the recommended three or four weeks between shots], it’s still worth getting the second dose,” says Dr. Brewer. “Even if it’s an extra month later or more, it’s to your benefit and the benefit of the community to go and get that second dose.”

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