What Happens if You Miss Your 2nd COVID-19 Shot?

While most people who received their first dose of the Pfizer or Moderna COVID-19 vaccine also received the second—95.6% among all two-dose recipients as of April 2021—the Centers for Disease Control and Prevention (CDC) said that there were still millions of people who avoided getting their second dose (the first two shots are referred to as the primary series vaccines, per the CDC).

As of July 2022, there were three vaccines with a two-dose primary series available for use in the US, per the CDC: Pfizer, Moderna, and Novavax. The Johnson & Johnson (J&J or Janssen) vaccine had one dose in the primary series (more on that later). Pfizer and Moderna booster shots were also available and could be combined with either of the four vaccines.

There were multiple reasons why people skipped their second shot. According to a July 2021 study published in the New England Journal of Medicine, researchers found that people were simply confused regarding the effectiveness of needing a second dose of the vaccine—they thought that one dose should protect them. People were also confused about the timing of the second dose, according to the same study.

Researchers of the July 2021 study stated that while most (85%) vaccinated people polled recalled being told that they needed a second dose, just 54% recalled being told that protection was strongest after the second dose. Researchers suggested that the combination of confusion around needing a second dose, as well as the timing of it probably contributed to many not going for their second shot—or being late getting it.

In a May 2021 UC Davis Health article, the authors stated that some people also feared the side effects of the second shot, such as flu-like symptoms, including fatigue, muscle aches, headache, and fever. This was enough to prevent some people from going for their second shot. Here's what you need to know about missing that important second shot.

What Happens if I Miss My Second Shot?

One shot of the Pfizer, Moderna, or Novavax vaccine does give you some protection from COVID-19, but the risk of getting it is lowest if you get both shots. The CDC confirmed that two doses are better than one, with a March 2021 study of almost 4,000 healthcare personnel, first responders, and front-line workers between December 14, 2020, and March 13, 2021.

The CDC found 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 virus variants, Ahmad Kamal, MD, MSc, clinical associate professor of medicine at Stanford University School of Medicine, told Health. "This is because a different type of immune response is stimulated by the second shot," Dr. Kamal explained.

The complete vaccination series also protects against more severe COVID-19 infection—the symptomatic infection that requires hospitalization, intensive care unit (ICU) admission, or ER visits—across all age groups, per a May 2021 study published in the journal Lancet (which studied the Pfizer vaccine) and an October 2021 study published in the New England Journal of Medicine (which studied mRNA vaccines more generally).

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, told Health.

What if I'm Late Getting My Second Shot?

Some people might simply forget to go for their second shot. In this case, get it as soon as you can, Dr. Parikh advised. "You'll get the same benefit from both shots, but you should get it ASAP, so you don't get sick between doses," Dr. Parikh said.

The CDC-recommended vaccine schedule is based on the clinical trials that initially led to emergency use authorization (EUA). "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 Health in Long Island, told 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," Dr. Krilov said.

What if My Pharmacy Has Run Out of Shots?

It's best to get both shots of your primary series of the Pfizer, Moderna, or Novavax vaccine from the same location because your COVID-19 vaccination records will be available there. But if your provider runs out of doses, nothing is stopping you from going elsewhere.

It's a good idea to call ahead to check that they have the right vaccine since the CDC recommended that the second shot in your primary series be the same brand (Pfizer, Moderna, or Novavax) as your first shot.

It's also a good idea to have your vaccine card with you as it, too, 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 said.

Even though 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 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, told Health.

What if I Only Want One Shot?

No one likes getting shots, so you may be tempted to choose the single-dose J&J vaccine over Pfizer, Moderna, or Novavax. But the US Food & Drug Administration (FDA) limited the use of the J&J vaccine in May 2022 due to possible rare but serious side effects.

Possible rare side effects of the J&J vaccine include thrombosis with thrombocytopenia syndrome (TTS), a serious condition that can cause blood clots, and Guillain-Barré Syndrome (GBS), a rare disorder that can cause muscle weakness and even paralysis, per the CDC.

You may consider the J&J vaccine in the following cases, per the CDC:

• If you've had a severe reaction after an mRNA vaccine (such as Pfizer or Moderna) or a protein subunit vaccine (such as Novavax), or if you're severely allergic to an ingredient in the other vaccines
• If you have limited access to Pfizer, Moderna, and Novavax vaccines and can't get one of those shots
• If you want to get the J&J vaccine despite safety concerns

Regardless of which brand you chose for your primary vaccine series, the CDC recommended a booster dose for most people for all vaccine brands, including the J&J and Novavax vaccines (as of July 2022, there were no Novavax-brand booster shots for use in the US).

Of note: As of July 2022, the CDC recommended that your primary vaccine series be the same brand, but the agency said it was OK to use a different brand (Pfizer or Moderna) for your booster shot. They also recommended either the Pfizer or Moderna shot be given for the J&J booster shot for most people.

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.