Tips to Live By

Viral Vector COVID-19 Vaccines: 4 Things to Know

March 2, 2021 - Katie McCallum

After a temporary pause, use of the Johnson & Johnson COVID-19 vaccine has resumed. However, the CDC has stated that women under the age of 50 should be aware of the rare but increased risk of experiencing an adverse event called thrombosis with thrombocytopenia syndrome (TTS). This risk has not been seen with the other COVID-19 vaccines available in the U.S., Pfizer and Moderna.

When the Pfizer and Moderna COVID-19 vaccines were made available back in December 2020, most of us hadn't heard of mRNA vaccines before, so we learned about them.

Now, we have another type of vaccine to familiarize ourselves with — viral vector vaccines. That's because on Feb. 27, 2021, the FDA granted Emergency Use Authorization (EUA) to a third COVID-19 vaccine: Johnson & Johnson's viral vector COVID-19 vaccine.

The Johnson & Johnson vaccine was shown to be more than 70% effective against preventing severe disease in the U.S. and 64% effective in South Africa where a prominent COVID-19 variant is circulating.

While this type of vaccine may sound new, viral vector vaccines aren't actually all that new.

"Viral vectors are a type of biological technology that have been used in science and medicine since the 1970s. Most recently, viral vector vaccines have been used to control Ebola outbreaks, and several others are currently being studied to fight influenza, HIV and Zika," says Dr. H. Dirk Sostman, chief academic officer of Houston Methodist. "Additionally, the Oxford-AstraZeneca COVID-19 vaccine that has been approved in many countries is also a viral vector vaccine."

With the first viral vector COVID-19 vaccine now available, it's important to understand how Johnson & Johnson's vaccine works and why it's safe, should it be offered to you when it's your chance to be vaccinated.

1. How viral vector COVID-19 vaccines work

Similar to the mRNA vaccines, viral vector COVID-19 vaccines use genetic material to help train your immune system to recognize the spike protein found on the surface of the coronavirus and respond accordingly. This means that if you're exposed to the coronavirus in the future, your body is well-equipped to fight it off.

There are two distinct differences with how viral vector COVID-19 vaccines work, however.

"The first difference is that the genetic material in this vaccine is DNA, rather than messenger RNA," says Dr. Sostman. "But not to worry, this small piece of DNA is harmless, degraded once its job is complete and cannot affect your own DNA in any way."

The second difference is how this genetic material is packaged. Instead of an artificial lipid shell, the DNA of a viral vector vaccine is carried within a harmless adenovirus.

"This adenovirus is modified such that it cannot cause illness or harm you. Its protein-based coat is merely used as a mechanism for delivering the DNA that will help mount your immune response against the coronavirus," explains Dr. Sostman.

2. Viral vector vaccines overcome cold storage challenges

While the Pfizer vaccine can only be kept in a refrigerator for about 5 days and the Moderna vaccine for about 30 days, the Johnson & Johnson vaccine can be refrigerated for up to 3 months.

This will likely aid in vaccine distribution, especially in rural areas.

"The reason these vaccines are more stable comes back to how they're different from mRNA vaccines. DNA itself is more stable than mRNA. In addition, the modified adenovirus carrying this DNA has a tough protein-based coat that is much more sturdy than the lipid shell used to package the mRNA in mRNA vaccines," explains Dr. Sostman.

3. Some viral vector vaccines require only a single dose

One of the most exciting things about the viral vector COVID-19 vaccine from Johnson & Johnson is that you only need one dose.

"While both the Pfizer and Moderna mRNA vaccines require two doses separated by several weeks, the Johnson & Johnson viral vector vaccine requires only a single shot," says Dr. Sostman.

A single dose not only makes things easier on you, but it will likely also help speed up distribution of this vaccine.

"Requiring only a single dose isn't a general feature of viral vector vaccines, however. This advantage is specific to the Johnson & Johnson COVID-19 vaccine. Other viral vector COVID-19 vaccines, such as the one developed by Oxford-AstraZeneca, still require two doses," adds Dr. Sostman.

That being said, you may eventually need a booster dose of the J&J vaccine to help prolong protective immunity. Anyone who is 18 years of age or older and received the J&J vaccine at least two months ago is eligible for a second dose of the vaccine.

4. Any vaccine granted EUA will have undergone thorough review

Just as the Pfizer and Moderna vaccines underwent careful review before being granted EUA, so has Johnson & Johnson's viral vector COVID-19 vaccine.

Not only was this vaccine rigorously tested for safety and effectiveness via clinical trials, the trial data was then scrutinized by several panels of experts. And while it may seem like this vaccine was developed quickly, know that testing and safety were not sacrificed for speed.

"All in all, before being made available to the public, Johnson & Johnson's viral vector COVID-19 vaccine went through preclinical testing, three phases of clinical trials and external review by two independent panels of experts and two civil service scientific reviews," explains Dr. Sostman.

Lastly, with three vaccines now available, you may be wondering which COVID-19 vaccine is best.

"The demand for these vaccines still far exceeds the supply, and each vaccine provides strong protection against severe illness, hospitalization and death from COVID-19. Given this, myself and other members of the Houston Methodist Vaccine Scientific Advisory Committee recommend taking the first COVID-19 vaccine made available to you," adds Dr. Sostman.

Of note, however, is the pause this vaccine underwent due to a rare and severe type of blood clot reported in a small number of vaccine recipients, particularly adult women younger than 50 years old. While the vaccine pause has since been lifted, the CDC has stated that, "women younger than 50 years old should be aware of the rare but increased risk of this adverse event and that there are other COVID-19 vaccine options available for which this risk has not been seen."


Sign up for your COVID-19 vaccine:

All individuals age 5 and older are currently eligible to receive a COVID-19 vaccine in Texas.


This article was updated on Nov. 8, 2021 to reflect the current state of the COVID-19 vaccine landscape.

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