Why Do You Need Two Doses for COVID-19 Vaccines

This guide will help explain why you need two doses for COVID-19 of the Pfizer-BioNTech and Moderna vaccines, and will provide answers to many other questions, too.

COVID-19 is the disorder caused by the novel coronavirus, SARS-CoV-2. When many people with COVID-19 get moderate symptoms, others can become severely sick. Several vaccines have been developed to safeguard against COVID-19.

The Food and Drug Administration (FDA) has up to now approved three COVID-19 vaccines for emergency usage. Both of these vaccines need two doses.

The Johnson & Johnson vaccine has also been approved for emergency use. However, this vaccine simply requires one dose.

Why do some experiments require 2 doses? How long do you need to wait between doses? Have you got any resistance following the first dose?

How do these vaccines work?

The Pfizer-BioNTech and Moderna vaccines both use mRNA technology. The cells within your body naturally use mRNA all the opportunity to create many diverse types of proteins which you need to remain healthy.

The mRNA from the two-dose COVID-19 vaccines supply the cells of the body with instructions on how to produce the spike protein that’s specific to the new coronavirus. This spike protein is located on the surface of the virus. It employs this spike protein to attach and enter a host cell inside your body.

If you get your COVID-19 vaccine, your immune system processes information about the spike protein and creates an immune reaction to it. This includes the production of antibodies.

Vaccines hinge on a very important aspect of your immune system, which is that it has memory. After your vaccination, your immune system continues to store info on the spike protein.

This may help keep you from becoming sick with COVID-19.

The entire immunity procedure typically takes about 2 weeks following the second dose of vaccine. That is why you can still contract the new coronavirus and become sick if you’re exposed to it soon after getting your own vaccine.

Why do the Pfizer-BioNTech and Moderna vaccines require two doses?

During ancient studies, researchers found that the Pfizer-BioNTech and Moderna vaccines provoke a relatively weakTrusted Source immune reaction when given as only 1 dose. But, there was a more powerful immune response when a second dose was inserted.

Basically, the first dose of the vaccine begins the procedure of building up protection. The second dose functions to greatly reinforce this coverage.

Here’s an analogy to help explain this: You and a friend are trying to move a heavy table across a room. Between the two of you, you are in a position to get it partway there. Then, another couple of friends jump in to assist, and you’re all able to move it the remainder of the way.

Vaccines that need more than 1 dose aren’t that uncommon.

  • That the measle-mumps-rubella (MMR) vaccine
  • vaccines against hepatitis A and hepatitis B
  • the shingles vaccine

Are there other COVID-19 vaccines that require two doses?

A lot of different varieties of COVID-19 vaccines, including ones still under development, additionally require two doses. Some examples include:

  • Oxford-AstraZeneca: 2 doses given 8 to 12 weeks
  • Novavax: two doses given 3 weeks
  • Sputnik V: 2 doses given 3 months
  • Coronavac: 2 doses given 1 month

Do some COVID-19 vaccines only require one dose?

The COVID-19 vaccine that has been developed by Johnson & Johnson only requires one dose.

After reviewing effectiveness and safety information from large-scale clinical trials, the FDA has given this vaccine Emergency Utilization Authorization from the USA.

How long do you need to wait between vaccines?

For both the Pfizer-BioNTech and Moderna vaccine, there’s a waiting period between the first and second dose. The timeframe between the doses depends on which vaccine you received.

Pfizer-BioNTech. You get your second dose 3 weeks (21 days) after your first dose.
Moderna. You get your next dose 4 weeks (28 days) after your first dose.

Why do you need this waiting period?

The very first dose helps to prime your immune system. It introduces it to the spike protein and allows it to create a small immune response prior to the second dose. Time is required to allow this procedure to develop properly.

It’s also important not to get your second dose too early, because this may impact the effectiveness of the vaccine.

According to recommendations from the Centers for Disease Control and Prevention (CDC), you can receive your second dose up to 4 times earlier than recommended.

What happens if you wait for a longer period of time to get your second vaccine?

The CDC recommends that you get your next dose as near the necessary period as you can.

You could be wondering what happens if you can not get your second dose within the specified timeframes. The CDC has guidanceTrusted Source for this too. In such scenarios, the next dose can be awarded up to 6 weeks (42 days) following the first dose.

We now don’t understand if delaying the next dose longer than this time has an impact on immunity.

If at all possible, try to schedule your appointment to your next dose on the day you receive your first dose. If you didn’t schedule your next dose right now, contact the location where you obtained your first dose to make an appointment.

What level of immunity do you have after the first vaccine?

Even though the Pfizer-BioNTech and Moderna vaccines need two doses, so you really do get some resistance once you get the first dose. Let us examine this in a bit more detail below.


However, the Pfizer-BioNTech vaccine may be more effective after the first dose than was formerly believed. This advice comes from another analysis from scientists in the United Kingdom.

These scientists wanted to look at vaccine efficacy 15 days or more after individuals had received the first dose of this Pfizer-BioNTech vaccine. They found that vaccine effectiveness was really closer to 89 to 91 percent 15 days following the first dose.


In a report submitted to the FDA, researchers examined vaccine efficacy in clinical trial participants who’d only gotten one dose of this vaccine.

A vaccine effectiveness of 50.8 percent was discovered up to 14 days after the first dose. Vaccine effectiveness was 92.1 percent past the first 14 days.

What level of immunity do you have after the second vaccine?


Researchers working on the Pfizer-BioNTech vaccine clinical trial appeared at vaccine efficacy 7 days following the second dose. Now, vaccine effectiveness was observed to be 95 percent.

And what about longer-term resistance? Another study, now in preprint, looked into this. This analysis followed people for 63 days when they’d received their second dose.

The researchers detected a strong antibody response following the next dose. While the overall levels of antibodies began to decrease gradually as time passed, they still remained high in the 63-day markers.


Researchers working on the Moderna clinical trial assessed vaccine efficacy 14 days after diagnosis participants had gotten their second dose.

Vaccine efficacy was 94.1% at this point.

Researchers also published another study of the immune response to this Moderna vaccine 90 days after the next dose. This analysis was rather small, involving 34 participants.

Elevated levels of antibodies were made following the next dose. Similar to this Pfizer-BioNTech study discussed above, antibody levels started to slowly decrease over time. However, immunity persisted at high levels at the 90-day mark.

Is the second dose necessary?

Because both vaccines appear to have high effectiveness 14 days or longer after the first dose, you may be thinking about whether the next dose of this vaccine is actually necessary.

It is correct that there have been discussions amongst public health scientists and officials about the possibility of delaying or eliminating the second dose of these vaccines.

It would also aid in extending vaccine supplies to more people.

However, there are some concerns about doing this. It’s possible that overall immunity or vaccine efficacy could be affected. Additionally, lower levels of resistance could drive the development of more virus variations that can escape our existing vaccines.

Further research to delaying or removing second doses is needed. Currently, the FDA strongly recommends continuing to adhere to the dosing schedule that was tested in clinical trials and authorized for emergency use.

Can you get Pfizer for one dose and Moderna for the other?

The CDC notes that though there is no preference for a single vaccine over another, the two vaccines aren’t interchangeable. As a result of this, it is important for your second dose from the exact same product as your first dose.

This is only one reason why COVID-19 vaccine record cards are provided after you get your first dose. These cards give invaluable information about which vaccine you received along with the date on which you obtained it.

Is there a difference in the side effects between the first and second dose?

The two-dose COVID-19 vaccines have similar negative effects, such as:

  • Injection site reactions, that may include soreness, redness, or swelling
  • fatigue
  • fever
  • chills
  • headache
  • body aches and pains
  • nausea
  • swollen lymph nodes

Side effects that are felt through the entire body, such as fever, fatigue, and headache, are somewhat more common after getting the next dose of the vaccine. This can be true for both the Pfizer-BioNTech and Moderna vaccines.

Remember that the very first dose of the vaccine creates a weaker response than the next dose. Because of this, you’re more inclined to feel side effects following the next dose.

These side effects are actually a sign that an immune reaction is taking place within your system. However, it’s also normal not to have any side effects at all. The vaccine will still be boosting your immune response, even without signs.