Vaccines are nothing new or unfamiliar — many people get them from the time they’re children. They protect against diseases like measles, polio, the flu, and now COVID-19. The Pfizer and Moderna COVID-19 vaccines add a new mRNA technology to a long-standing practice of immunization.

Here’s how those novel coronavirus vaccines compare to others:

How they work

Joseph Kontra, chief of infectious diseases at Penn Medicine Lancaster General Health, explains that the mRNA — or messenger RNA — technology used in the COVID-19 vaccines has been studied for decades. “The technology for using messenger RNA goes back to the mid-1990s,” he notes.

The vaccines work by introducing a synthetic piece of mRNA into the body. “That messenger RNA basically is a chemical recipe, and it’s a recipe for making the spike protein, which is the protein that covers the surface of the COVID-19 virus,” explains Kontra.

The mRNA briefly enters the cells in the body and produces the spike protein, which the body recognizes as foreign and fends off by producing antibodies. “The next time you see the spike protein, this time perhaps attached to a virus, then your body’s able to react to it within hours and create a robust antibody response,” Kontra says.

The COVID-19 vaccines are the first to work this way. Others work by introducing weakened live viruses into the body, as the measles vaccine does, or by introducing pieces of a virus, like the flu shot does.

“Messenger RNA is new, but it works in a similar fashion to others, except it gives you the message to make the proteins instead of giving you the proteins,” Kontra explains.

Development and approval

The way the COVID-19 vaccines work may be different from other vaccines already in existence, but the research and approval process followed to develop the novel coronavirus vaccines was the same as it has been for “every other vaccine that’s been developed,” says Kontra.

“Companies begin with their scientific research and development mechanism to make the vaccine,” Kontra says.

The vaccine then undergoes three phases of study. The first two phases focus on the kind of antibody response the vaccine triggers as well as any harmful effects of the medication.

The phase three studies focus more on efficacy, explains Kontra. During this phase, participants are given either the real vaccine or a placebo, and researchers look at how many people get the disease in the vaccine group versus the placebo group.

Throughout the studies, an independent data and safety monitoring board oversees the research process and can call a halt to the work if “anything looks awry,” Kontra says. There is also an FDA committee that reviews all data from the studies and decides whether to approve the vaccine.

Finally, the Advisory Committee on Immunization Practices (a subset of the CDC), offers “final recommendations as to who should get the vaccine,” as well as dosing recommendations, Kontra explains.

“That entire process was present for the COVID-19 vaccines,” says Kontra. The reason the process was much faster for the coronavirus vaccine than it has been for other immunizations was that Operation Warp Speed provided financial assistance for the companies developing the vaccine, Kontra says.

“In some vaccines, it may take many years to create a vaccine, not because you can’t do it in a shorter time but because the company has to invest a lot of money to get the vaccine up and running,” says Kontra.

He adds, “That plus the combination of the unique science behind the messenger RNA vaccines were the reasons why this seemed to go faster than usual. But, in fact, all the safety stops were there.”

Side effects

“Each vaccine has its own unique array of side effects,” says Kontra. He says the Pfizer and Moderna COVID-19 vaccines are considered “reactogenic, meaning that they do create some symptoms.”

Kontra says a common side effect of the coronavirus vaccines is a sore arm. There can also be a bit of a “flu-like illness” experienced including muscle aches, headache and fatigue, especially after the second dose, he notes.

One epidemiologist says the side effects of the COVID-19 vaccines are similar to those of the shingles vaccine.

Kontra says the symptoms individuals may experience after getting the COVID-19 vaccines are “direct proof that the vaccine is actually working in your body.”

The more serious side effects of the COVID-19 vaccines are very uncommon, experienced by “far less” than 1% of people, Kontra adds.


The Moderna and Pfizer vaccines are about 95% effective at preventing COVID-19 after the second dose, says Kontra. “I think most vaccinologists would have been dancing in the streets at 70% effective,” he says.

For comparison, the CDC reports that the flu vaccine was 39% effective in the 2019-2020 season. In the last 11 flu seasons, dating back to 2009, the flu vaccine has ranged from a high of 60% effective to a low of 19%.

Another common inoculation, the MMR (measles, mumps, rubella) vaccine is 97% effective against measles and 88% effective against mumps after the second dose, according to the CDC. The vaccine’s effectiveness against measles is important, says Kontra, “because measles is the most contagious disease on the planet.”


Like other vaccines, including those for MMR and chickenpox, the Pfizer and Moderna COVID-19 vaccines require two doses for maximum effectiveness. Kontra says that for individuals getting vaccinated, both of these two shots will likely be given at the same immunization clinic “because there’s more than one vaccine out and you don’t want to mix them.”

“The second dose is the same as the first dose, except that it now is reminding your immune system of that immunity that was created after the first shot,” explains Kontra, “and the maximum immunity comes about a week or so after that second immunization.”

Coronavirus vaccine distribution has been slower and rockier than many had hoped. Kontra says a major goal of Operation Warp Speed was “to encourage manufacturers to work together to produce vaccine in ginormous quantities.”

However, Kontra says, “The problem is that even the magnitude of production that was anticipated is…falling short of what’s actually needed in the short term.”

Although producing and distributing vaccines is not an uncommon undertaking, Kontra says that the urgency of COVID-19 vaccinations in response to a pandemic makes this situation fairly unique. Other vaccines, like the polio vaccine, have been produced in response to major health crises.

A History article reports that the first polio vaccine faced shortages after its approval. The article states that only enough doses of the vaccine had been procured initially “to target the youngest and most vulnerable kids” while everyone else “would have to wait through another anxious summer of pool closures and social distancing.”

Whether updated doses of the COVID-19 vaccine will be needed to target new strains of the virus, as is the case with yearly flu shots, is still unknown, says Kontra.