According to the most recent data from the Centers for Disease Control and Prevention (CDC), more than five million Americans have missed their second doses of the Pfizer/Moderna COVID-19 vaccines as of April. That’s almost 8% of those who got the first shot—up from about 3.4% back in March.

The CDC said the increase was to be expected, as access and eligibility increased, reported ABC News. However, experts are concerned that too many people aren’t getting fully inoculated.

There are multiple reasons for this; according to The New York Times, some people said in interviews that they feared the side effects, such as flu like symptoms, while others said they felt a single shot was enough protection against the virus. In other cases, vaccine providers were forced to cancel second-dose appointments because they didn’t have enough supply or the right brand in stock. In any case, the situation is troubling—especially if the US is hoping to reach herd immunity to combat the virus. Here’s what you need to know about missing that very important second shot.

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What happens if I miss my second shot?
One shot of the Pfizer or Moderna vaccine does give you some protection from the coronavirus, but the risk of getting COVID-19 is lowest if you get both shots. The CDC recently confirmed that two doses are better than one, with a study of almost 4,000 health-care personnel, first responders, and front-line workers between December 14 and March 13 finding that one dose of the Pfizer or Moderna vaccine conferred 80% protection, vs. 90% two weeks after the second dose.

Plus, the protection from one shot may not last as long as the protection from both shots, particularly against some of the newer virus variants, Ahmad Kamal, MD, MSc, vice chair of internal medicine at Santa Clara Valley Medical Center in San Jose, California, tells Health. “This is because a different type of immune response is stimulated by the second shot,” he says.

And remember: Even after your second dose of the Pfizer or moderna vaccine, you’re not fully protected from COVID-19 until two weeks after that, Purvi Parikh, MD, who specializes in infectious disease allergy and immunology at NYU Langone Health, tells Health.

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What if I’m late to get my second shot?
Some people might simply forget to go for their second shot. In this case, simply get it as soon as you can, advises Dr. Parikh. “You’ll get the same benefit from both shots, but you should get it ASAP so you don’t get sick between doses,” she says.

The recommended vaccine schedule is based on the clinical trials that led to the present emergency use authorization (EUA) approval. “It’s not possible to determine whether delaying the second dose would impact the extent of the immune response, since other dosing schedules were not studied,” infectious disease specialist Leonard Krilov, MD, from NYU Langone Hospital—Long Island, tells Health. “That said, an interval up to 90 or 120 days between doses would probably still be effective and even if it is longer, the second dose should be given.”

It’s absolutely fine to reschedule your second COVID-19 shot with your provider—they won’t turn you away just because you missed your appointment.

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What if my pharmacy has run out of shots?
It’s best to get both shots of the Pfizer or Moderna vaccine at the same place because all your records will be available. But if your provider runs out of shots—and this is unlikely now that supply levels have improved, Dr. Kamal points out—there’s nothing stopping you from going elsewhere. It’s a good idea to call ahead to check that they have the right vaccine since your second shot should be of the same brand (Pfizer or Moderna) as your first shot.

And make sure you have your vaccine card with you as it has all the information about your first dose. If you’ve lost or misplaced your card, the provider can look you up in your state’s database, but this may create further delay, Dr. Kamal says.

While the same vaccine brand (Pfizer or Moderna) should be consistent for each dose, there’s no reason to think that if given in different pharmacies or clinics there would be a problem as long as storage and administration guidelines are followed, adds Dr. Krilov.

While it may be the preferred and most convenient way to get your second dose from the same vaccine provider as your first, it is not required that you do so. Many of the scheduling websites now include questions to identify which dose of which vaccine you are seeking as part of making your appointment. “I know a lot of people who have received their two doses of vaccine from two different vaccine providers,” Richard Seidman, MD, MPH, Chief Medical Officer of L.A. Care Health Plan, the largest publicly operated health plan in the country, tells Health.

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What if I only want one shot?
Listen, no one likes getting shots—and if you’re very focused on only having to get stuck once, there’s a simple solution: seek a provider who gives the single-shot Johnson & Johnson (aka Janssen) vaccine.

Though the Johnson & Johnson COVID-19 vaccine recently underwent a recommended pause due to six cases of a rare and severe type of blood clot seen in patients—mainly women—who were recently inoculated, the US Food and Drug Administration (FDA) and CDC lifted that pause on April 23, giving the OK to providers to resume doses.

Both the CDC and FDA maintain that the Johnson & Johnson vaccine is safe and effective in preventing COVID-19, and that the chance of developing the rare type of blood clot—which providers are referring to as thrombosis-thrombocytopenia syndrome (TTS)—is very low.

Overall though, according to Dr. Seidman, the Johnson & Johnson vaccine is the best option for anyone who only wants to get one shot of any COVID-19 vaccine—not only for them, but for their loved ones and community.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it’s possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.