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Comparing COVID Vaccines: Side effects, how long they last, and the science behind them

Megan Coffee, MD
Crossover Physician
April 7, 2021
NOTE: This blog was written and posted prior to the J&J statement that was released on April 13th, 2021, which details the pause in their vaccine distribution and usage.

COVID vaccines are a bit of light at the end of this pandemic tunnel. These vaccines have been tested rigorously with all eyes on them. The vaccines approved by the US FDA have been shown to be really effective at stopping you from getting sick from COVID.

What are the different types of COVID vaccines and how do they work?

There are two main types of vaccines being used now in the US. 

The first to be approved were the two mRNA vaccines—Pfizer and Moderna. These vaccines are a bit like the Snapchat of vaccines. To give a little background, Snapchat messages are sent, and within a certain amount of time, they disappear.  Like the code of Snapchat messages, the mRNA vaccine messages don’t last very long—they just go from where they are injected, straight over to the closest lymph node (and nowhere else), and then a short while later, they disappear. 

Before disappearing though, when the vaccine gets to the lymph nodes, it essentially delivers a message that includes an image of the spike proteins that are found on the outside of the COVID virus. The cells in our lymph nodes take a picture of the spike protein image before it goes away. From there, our immune system memorizes that picture so it knows what to look for in our body. So, once you have received the vaccine and your immune system is on the lookout, if you are exposed to COVID, your body will recognize it and react quickly. Side note: none of the vaccines can give you the COVID virus or disease.

The Johnson and Johnson vaccine sends a similar message code of a spike protein to our lymph nodes—but it delivers it using a different app. It’s still like a Snapchat message, in that it won’t last long, but instead of being delivered via a mRNA, a virus carries the code. This virus is nothing like COVID—it’s a lot like the cold viruses we have had in the past, minus the symptoms. This cold virus carries the message code to cells in our lymph nodes, and our immune system then memorizes how the spike protein built from the code looks so it can recognize the COVID virus if it ever arrives. Then, the virus that brought the message disappears.

The vaccine designers have studied how the virus that causes COVID (and similar viruses) enter into our cells. The COVID virus infects us by first entering into the cells in our noses, our lungs, and elsewhere. Those cells are the building blocks that make us uniquely who we are—and what scientists have recognized is that as the virus that causes COVID enters our cells, the spike protein on its outside shape-shifts a little bit, bending into a different shape. This new shape makes the best “Wanted Photo” for our immune system so it knows exactly what the COVID virus looks like when infecting and attacking us.

But both of the mRNA vaccines and the Johnson & Johnson vaccine do something clever. They don’t use the exact spike protein from the virus. Instead, these vaccines use tweaked versions of the spike protein that mimic its appearance as it enters our cells. This helps train our immune system to stop the virus causing COVID right before it enters the cells, infecting us. That’s perfect timing. This clever tweak is thought to help create a strong memory and may help some vaccines fight different types, or new COVID variants, more effectively. This work was all built on our prior knowledge of similar viruses (like one called MERS—Middle East Respiratory Syndrome). These past learnings (along with dedicated resources) have allowed scientists to make effective COVID vaccines quickly, without cutting any corners in testing or safety.

There’s another vaccine we may hear more about soon as it is evaluated for use—the Novavax vaccine. This vaccine sends the spike protein over directly, with no code or messages to create it. This protein comes into our bodies as a red flag that alerts our immune system that it needs to be remembered and recognized.

Is there one vaccine that’s best for me?

The best vaccine is the one in your arm—meaning any one of the COVID vaccines will do. We don’t recommend waiting to sign up for one vaccine over another. If someone is vaccinated abroad, they may have a different type of vaccine.

What are the side effects from the vaccines like?

The younger you are, the more likely you are to experience some side effects. You’ll probably have some arm pain where you had the injection. You may feel tired and have a headache. You might have the chills or feel hot. With the Moderna vaccine, you can sometimes have a red skin reaction at the site of the vaccine, which goes away in a few days. Even with these potential side effects, it’s important to get your second vaccine shot to be fully vaccinated. 

These symptoms don’t usually start right after you get the shot; if you have side effects, you’ll feel them a few hours (or the day) after the shot. If within 30 minutes or so of receiving the vaccine you have any shortness of breath, tingling or swelling in your throat or lips or tongue, wheezing, abdominal pain, or anything else that concerns you, ask the healthcare providers at the vaccine clinic to see you immediately. It is possible you could be having an allergic reaction and may need treatment.

Is there any reason I shouldn’t get the vaccine?

You should not take a vaccine if you’ve had a severe or immediate allergic reaction to this same vaccine (i.e., on the first dose) or any ingredient in it (especially polyethylene glycol). So if you haven’t had a COVID vaccine yet, but you’ve had allergic reactions to vaccines in the past, talk to your doctor about that before getting the COVID vaccine. If you have had a prior severe immune reaction (especially if diagnosed with anaphylaxis/you couldn’t breathe), talk to your doctor about the COVID vaccine and ingredients.

If you have an allergic reaction to your first dose of an mRNA vaccine, you should ask your physician if you should have a Johnson and Johnson vaccine for your next shot. If you had the Johnson and Johnson vaccine first, you don’t need a second shot.

Can I take this vaccine after I’ve had COVID?

Yep. Just make sure you are no longer infectious. You are no longer infectious 10 days after you first experience symptoms (or if you were asymptomatic, 10 days after your first positive test), and when you have had no fever for a day (without having taken Tylenol, ibuprofen, or Aleve), and when you feel your cough, breathing, and any other symptoms are improving. 

If you have had COVID, you may have more side effects with the first shot than others do, but it’s worth it. Natural immunity isn’t perfect and can decrease over time. With new variants spreading, vaccine immunity can make a difference when natural immunity doesn’t help. Some people who have symptoms that have lasted for months after having COVID have found their symptoms improved after vaccination.

What if I’m pregnant, breastfeeding, or trying to get pregnant?

You can still choose to be vaccinated. Many professional medical societies for pregnant women have stated that vaccines should not be withheld from pregnant or lactating women (or those trying to get pregnant). COVID can be more serious in pregnant women. However, the trials did not include pregnant women (though some women did become pregnant). After the vaccines were licensed, the CDC ran a program to track any side effects in pregnant women and they have not seen any that cause concern.

How long does the vaccine last?

That is to be determined. The first people to be vaccinated were vaccinated over a year ago. Larger numbers of people have been followed for up to six months, and with the Pfizer vaccine we have seen a strong continued immune response. That said, we may need boosters because of the different variants, or variations of the virus that causes COVID. Some of these variants do not respond as well to some vaccines.

When can I get vaccinated?

The odds are that a vaccine is available to you now. Most states are opening up vaccination to most adults. Vaccinations should be available for everyone over the age of 16 by May 1, 2021. 

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MEDICAL DISCLAIMER The information used for this purpose is not intended nor implied to be a substitute for professional medical advice, it is provided for health educational purposes only. You assume full responsibility for how you choose to use this information. Always seek the advice of your physician before starting any new treatment or discontinuing an existing treatment. Talk with your healthcare provider about any questions you may have regarding a medical condition. Nothing contained in these topics is intended to be used for medical diagnosis or treatment. If you reasonably believe you have an emergency medical condition, call 911 or go to the nearest emergency department. An emergency medical condition is a medical or psychiatric condition that requires immediate medical attention to prevent serious jeopardy to your health.
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