There are many different COVID-19 vaccines in development worldwide. In the United States, the Food and Drug Administration (FDA) has so far authorized three COVID-19 vaccines for emergency use.

Two of these vaccines are the Pfizer-BioNTech and Moderna vaccines, which use mRNA technology to help your immune system generate immunity to SARS-CoV-2, the coronavirus that causes COVID-19.

Both of these vaccines require two doses. It’s after receiving your second dose that your immunity to the virus fully kicks in. You may be wondering how long after the second dose you have full immunity.

In this article, we’ll take a deeper dive into what you need to know about immunity with the Pfizer-BioNTech and Moderna vaccines.

Both the Pfizer-BioNTech and Moderna vaccines work by introducing your immune system to a part of the new coronavirus called the spike protein. This protein is found on the viral surface. It’s used to help the virus bind to and enter host cells in your body.

Because your immune system has a memory, it can use the vaccine to analyze and store information about the spike protein. It can then draw upon this information to protect you if you’re exposed to the actual virus in the future.

However, immunity doesn’t happen immediately after vaccination. In fact, it typically takes about 2 weeks for your body to build up immunity. Because of this, you can still become ill during this time frame.

Now that we’ve discussed how long it generally takes to have immunity, let’s take a look at the effectiveness of the Pfizer-BioNTech and Moderna vaccines in the weeks after the second dose.


The Pfizer-BioNTech clinical trial evaluated vaccine effectiveness 1 week after participants had gotten their second dose. Researchers found that the vaccine was 95 percent effective at preventing COVID-19 at this point.


The Moderna clinical trial looked at vaccine effectiveness 2 weeks after participants had received their second dose. At this point, the vaccine was found to be 94.1 percent effective at preventing COVID-19.

The time period between the two doses depends on which of the two vaccines you get:

  • Pfizer-BioNTech: The second dose is given 3 weeks after the first dose.
  • Moderna: The second dose is given 4 weeks after the first dose.

During early testing, researchers found that both vaccines produced a weak immune response after just one dose. However, a much stronger immune response was observed following the second dose.

This is why two doses of the Pfizer-BioNTech and Moderna vaccines are needed. Think of it like this: The first dose starts building protection, while the second dose reinforces that protection.

There are some vaccines that only require one dose. An example of this is the vaccine developed by Johnson & Johnson.

This vaccine uses a different type of technology than the Pfizer-BioNTech and Moderna vaccines. After reviewing safety and effectiveness data from clinical trials, the FDA authorized the Johnson & Johnson vaccine for emergency use.

Some immunity is generated following the first dose of the vaccine. How this information has been reported is also different for the two vaccines.


For the Pfizer-BioNTech vaccine, an effectiveness of 52 percent was reported between the time of the first and second doses. However, vaccine effectiveness after the first dose may actually be higher than this.

A separate analysis by scientists in the United Kingdom estimated that vaccine effectiveness was closer to 89 to 91 percent 15 days or more after the first dose.

Additionally, a 2021 study of the vaccination campaign in Israel observed significant reductions in COVID-19 cases after one dose of the Pfizer-BioNTech vaccine.


A report submitted to the FDA detailed the effectiveness of the Moderna vaccine after only one dose. In this case, researchers found that vaccine effectiveness was 50.8 percent up to 14 days later and was 92.1 percent beyond this period.

The Centers for Disease Control and Prevention (CDC) currently recommends that you get your second dose as close to the 3-week (Pfizer-BioNTech) or 4-week (Moderna) waiting period as possible.

But sometimes, delays in the vaccine supply chain or unforeseen weather events may prevent you from getting your second vaccine right on time. Fortunately, there’s some wiggle room with this, and getting your second dose a little later than anticipated shouldn’t affect your immunity.

According to the CDC’s guidelines, it’s best not to receive your second dose more than 6 weeks after you received your first dose.

The impact of delaying the second dose any longer is unknown at this time.

Delaying or eliminating the second dose

You may have also heard discussion about intentionally delaying or even eliminating the second dose of the Pfizer-BioNTech or Moderna vaccines. What’s the thought process behind this?

We know that one dose of either vaccine can give some level of immunity. So, the idea here is that delaying or eliminating the second dose would allow more people to receive some protection while helping to stretch limited vaccine supplies.

However, the truth is that we currently don’t know what impact this would have on immunity. It’s possible, but not known, that vaccine effectiveness could be lower in this scenario.

Overall, further research into this topic is needed. Until we have further information, the FDA strongly recommends sticking to the dosing schedule that was tested within the clinical trials and authorized for emergency use.

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All COVID-19 vaccines have only been around for a very short amount of time. Because of this, it’s not known exactly how long immunity lasts after being vaccinated. This is true for both one-dose and two-dose vaccines.

Going forward, scientists will continue to study the different COVID-19 vaccines and how long their immunity lasts.

Even though we don’t know how long protection from COVID-19 vaccines lasts, it’s still very important to receive your vaccination when it’s available to you.

This is because getting vaccinated can prevent you from getting COVID-19. Even if you do get the disease, you’ll have a much lower risk for developing a serious or life threatening illness.

Over the past several months, new strains, or variants, of SARS-CoV-2 have been identified. There have been concerns about about how effective the vaccines are against these new variants. Two new strains you may have heard a lot about are:

  • B.1.1.7 (the “U.K. variant”)
  • B.1.351 (the “South African variant”)

Laboratory (test tube) studies have been performed with these strains and the Pfizer-BioNTech and Moderna vaccines. Initial data indicates that these vaccines may be less effective against the South African strain.

One report tested antibodies generated by the Pfizer-BioNTech vaccine against a test virus containing the spike protein mutations found in the South African variant. When compared to an early strain of the novel coronavirus, antibody neutralization of this virus was two-thirds weaker.

A similar report looked at the ability of antibodies generated by the Moderna vaccine to neutralize test viruses. While viruses from the U.K. variant were neutralized, researchers saw a 6.4-fold drop in neutralization for the South African strain.

It’s important to note that this is still a developing area of study. Researchers will continue to investigate the impact that emerging strains have on current and future COVID-19 vaccines.

If you’ve received both doses of the vaccine, it’s important to still continue to take precautions, including:

  • Wearing a mask. Wear a mask that covers your nose and mouth when you’re around people outside of your household.
  • Hand-washing. Hand-washing is particularly important after being out in public, after coughing and sneezing, and after using the bathroom.
  • Practicing physical distancing. Try to stay at least 6 feet away from people outside of your household.
  • Avoiding crowded areas. Places that are crowded or poorly ventilated can make it easier to contract and transmit the virus.

These precautions are important because we currently don’t know whether people who’ve been vaccinated can still spread the virus to others, even if they themselves don’t develop symptoms.

If you get the Pfizer-BioNTech or Moderna vaccine, you’ll need two doses. You generally have full immunity about 2 weeks after getting your second dose. It’s currently unknown exactly how long this immunity lasts.

While you get some immunity from the first dose, receiving the second dose greatly reinforces that immunity. Because of this, it’s important to get your second dose within the appropriate timeframe.

It’s unknown whether people who’ve been vaccinated can pass the virus to others. That’s why it’s important to continue to take precautions even after you’ve gotten both doses of the vaccine.