What's the difference between the COVID-19 vaccines?

The answer is complicated, but the bottom line is simple: Get one

Now that COVID-19 vaccines are making their way into the general community, many people are wondering which vaccine they should get when it’s their turn in line. The one that comes in one dose or two? 66 percent vs. 95 percent effective? One that uses mRNA or DNA?

The short answer is, it doesn’t matter. 

“The more important thing is to get a vaccine. It doesn’t matter which one. Just get it,” said Dixie Harris, MD, a pulmonologist at Intermountain Healthcare. “Any vaccine is better than no vaccine, according to the data I’m seeing.”

Efficacy: What do the numbers mean?

All the vaccines we’re hearing about in the news have shown tremendous promise. The vaccines, commonly referred to by their makers’ names, include drugs from:

  • Pfizer (currently in use);
  • Moderna (currently in use);
  • Janssen, from Johnson & Johnson (authorized for use)
  • AstraZeneca (in final clinical trials in the United States; already in use in Canada and the United Kingdom);
  • Novavax (in final clinical trials).

These vaccines have reported efficacy of between 66 and 95 percent in clinical trials. While that seems like a pretty big spread, it’s important to remember that numbers don’t tell the whole story.

Some of the vaccine trials measured the ability to prevent only serious illness, while others looked at success in preventing any signs of illness, including mild ones. Another difference is that earlier vaccines may not have been used on the newly emerging variants of the virus, while more recent trials have had to contend with more than one strain of virus. Stacking up their success rates is like comparing apples to oranges.

The most important thing is this: All the drugs have so far have shown a remarkable ability to prevent serious illness, hospitalization, and death from COVID-19, and that’s what really matters. 

Don't just look at the numbers

It’s important to look beyond the numbers, too. The Johnson & Johnson (J&J) vaccine becomes more protective as time passes from the date of vaccination. It was 86 percent effective at stopping severe cases of the disease 28 days after vaccination and 100 percent effective 49 days after vaccination. Most importantly, it has been 100 percent effective in stopping hospitalizations and deaths from the disease after vaccination. It also has the advantage of requiring only one dose and can be transported and stored more easily. The drug maker says it can produce one million doses by the end of June. And AstraZeneca has reported that its vaccine can substantially slow the transmission of the virus from vaccinated to unvaccinated people, which is a question researchers have been eagerly trying to answer for all the vaccines.

Speed is the key. The sooner we get COVID-19 under control, the less chance the virus has of mutating and becoming harder to contain. That's another reason to take the vaccine as soon as it’s available to you, regardless of the maker.

New strains

One question that remains is whether these vaccines will work on the new strains of the virus that are emerging around the world. The J&J vaccine was studied in South Africa, where most of the cases are due to the new B151 strain of the virus. 

“It absolutely looks promising in preventing severe disease in that South African variant, which is really welcome news," said infectious disease physician Eddie Stenehjem, MD.

We don’t yet know if the other vaccines can stop new variants in their tracks, but early investigations suggest they are effective at preventing severe cases, hospitalizations, and death. The Food and Drug Administration (FDA) is gathering data now, and vaccine makers are already at work on new tools to fight new variants.

Safety concerns

The five vaccines above work in different ways. 

  • Pfizer and Moderna are both mRNA-based, which means they use messenger RNA to teach our cells to create a harmless piece of protein called a “spike” protein that spurs an immune reaction. This type of mRNA vaccine is new, but the science behind it has been in development for years. The FDA granted emergency use authorizations (EUAs) for these vaccines in December 2020.
  • The J&J vaccine takes a harmless form of a virus, such as the ones that cause the common cold, and engineers it to teach our mRNA to form the spike protein. This is called a viral vector vaccine, and the technology has been used against the flu, Ebola, and Zika.
  • AstraZeneca takes a weakened form of the virus that causes the cold and modifies it to look like COVID-19, triggering an immune response. This is also a viral vector vaccine.
  • Novavax works differently than the other four. Instead of teaching our bodies to create the spike protein, this vaccine already contains the spike protein, along with an “adjuvant” that triggers the body to mount a response to it. This kind of protein vaccine is quite common and is used in vaccines to fight hepatitis B, diphtheria and tetanus. 

The mechanisms that make each vaccine work are highly complex, but one thing is simple. Any vaccine authorized for use in the United States goes through the same rigorous testing and approval process. You won’t receive any vaccine if the researchers, FDA, and Centers for Disease Control and Prevention (CDC) aren’t convinced of its safety. 


One last consideration is supply and timing. While the United States ramps up the number of doses available, we probably won’t have the option of vaccine shopping. Don’t let that stop you from getting the vaccine  whichever it is  as soon as it’s available. It’s the surest way to protect ourselves and our community.