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Why are Two Doses of the COVID-19 Vaccine Important?

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hand with writing that says, "second dose"; Why are Two Doses of the COVID-19 Vaccine Important?

Whether you’re worried about side effects or, simply, life just happened and you missed your second COVID-19 dose, our expert says it’s not a good idea to go without finishing the vaccine. Dr. Stephen Rinderknecht, chair of the UnityPoint Clinic vaccine committee, explains why you need two doses of the mRNA COVID-19 vaccine and what you need to know to get back on track.

Let’s Clarify – Who Needs a Second Dose?

There are several COVID-19 vaccines in use. The mRNA vaccines (Pfizer and Moderna) require two doses for the best and most effective protection against the virus. There is one viral vector vaccine by Johnson & Johnson/Janssen, which only requires one dose. If you received the J&J vaccine, you don’t need a second dose. If you received an mRNA vaccine (Pfizer or Moderna), two doses are required.

Is there Any Reason NOT to get the Second COVID-19 Vaccine?

Yes, there is one, and only one, reason someone who’s received the first mRNA vaccine shouldn’t get the second dose.

“The only reason not to get a second dose is if a person experiences an allergic reaction after the first dose. Allergic reactions are much different from side effects, which include the normal reactions of headache, tiredness, fever, chills, muscle pain, sore arm and even nausea. If you have any questions, talk to your health care provider,” Dr. Rinderknecht says.

Do You Need Two Doses of the COVID-19 Vaccine?

Yes, it’s important to get both doses of the COVID-19 mRNA vaccines because it’s what your immune system needs to adequately protect you from the virus. While you will have some immune response after the first dose, it’s not as strong and long-last as what you'll get after being fully vaccinated with both doses. To understand why completing the second dose is so important, Dr. Rinderknecht explains how our immune system works to protect us.

  • Primary Immune Response. When our body’s immune system first recognizes a substance (antigen) as foreign, it mounts a primary immune response. This response is the development of antibodies to fight off the invader as well as activation of memory T-cells. These antibody levels usually decrease over time, but memory T-cells persist much longer, often for years.
  • Secondary (or Anamnestic) Immune Response. When our immune system is confronted with the same substance (antigen) the next time, our memory T-cells kick in and we develop a secondary immune response. The secondary response produces antibodies much more rapidly, at a higher quantity and that are longer lasting than the primary response.

“The person who skips the second dose of a COVID-19 vaccine never develops that secondary immune response. They aren’t as protected when confronted with the natural disease. If you only get the first dose and not the second and then get infected by the virus, you will likely experience that secondary immune response. The problem is instead of a harmless carrier (in the vaccine), you are being confronted with the infectious organism. That could result in COVID-19 and its complications. It’s not worth the risk,” Dr. Rinderknecht says.

Are the First and Second COVID-19 Vaccine Doses the Same?

The amount injected into your muscle is the same during each dose – 0.3 mL (Pfizer) or 0.5 mL (Moderna). It doesn’t matter which arm you chose for your vaccine, and it can be a different arm each time. If you’re wondering why many people are experiencing more side effects after the second dose since it’s the same amount, Dr. Rinderknecht says it’s all based on what we just learned above.

“The secondary (anamnestic) response is also the reason we generally see more side effects after the second dose of the COVD-19 vaccine. The immune system reaction is greater. That doesn’t mean if you experience few or no side effects you are less protected. Just count yourself as lucky,” Dr. Rinderknecht says.

He suggests timing your vaccine, especially the second dose, when your schedule is more accommodating. 

“Remember, when you get COVID-19, you are in isolation for at least 10 days. So, even if you feel ill for a day or two with the COVID-19 vaccine, it’s much less of a burden than getting the actual virus,” Dr. Rinderknecht says.

When Should I Get the Second COVID-19 Vaccine Dose?

If you received one of the mRNA vaccines, here is the timing recommendation based on the initial study population:

  • Pfizer COVID-19 Vaccine. Get the second dose 21 – 42 days after the first shot.
  • Moderna COVID-19 Vaccine. Get the second dose 28 – 42 days after the first shot.

If you missed the time window for your second dose, it’s never too late to get it. Even if you passed the 42-day mark, you never need to repeat or restart the vaccine series.

What if I Can’t Remember the Details of My COVID-19 Vaccine?

“If you lose your vaccination card and don’t remember which product you received, try contacting the site where you got your vaccine for that information. They should have it on record because all doses are entered into the state’s vaccine information system,” Dr. Rinderknecht says.

The two mRNA vaccines aren’t interchangeable, so both doses of the series should be completed with the same product. The safety and efficacy of a mixed-product series haven’t been tested or evaluated at this time.

“In the exceptional situation where the product used can’t be obtained, either mRNA vaccine should be used as the second dose with at least a 28-day interval between doses. If the wrong product is ever accidentally given as the second dose, that dose should be counted as valid and doesn’t need to be repeated,” Dr. Rinderknecht says.