January 26, 2021

How COVID-19 vaccines work: Answers for you

20 million vaccinations and intense scientific scrutiny show vaccines are safe and effective

COVID-19 vaccines bring the promise of global light at the end of the pandemic, but many people have questions about the science, the effectiveness and the safety.

The vaccines are equally safe and effective for everyone, including minorities, seniors and those with underlying conditions. The vaccines are equally safe and effective for everyone, including minorities, seniors and those with underlying conditions.

Dean Blumberg, chief of pediatric infectious diseases at UC Davis Children’s Hospital and Natascha Tuznik, assistant clinical professor of infectious diseases, help answer questions about the vaccine.

Are the vaccines safe?

“Both vaccines have been scientifically proven to be very safe,” Blumberg said. “They went through full reviews by experts. There were no shortcuts.”

Nearly 20 million people have gotten at least their first of two doses, according to the Centers for Disease Control and Prevention (CDC) tracker, and there have been extraordinarily few reports of any serious reactions or problems.

Answers about the effectiveness of COVID-19 vaccines

The Pfizer and Moderna vaccines are among the most effective vaccines ever developed.

They are about 95% effective against COVID-19, which puts them in the same range as the measles, chickenpox and smallpox vaccines. In addition, they also reduce severity in almost all cases.

What does 95% effective mean?
That is an extraordinarily high rate of effectiveness, and it means the COVID-19 vaccines work very well. By comparison, flu vaccines are about 40% effective, on average.

Here’s where the number comes from: In the clinical trials, half the people got the coronavirus vaccine and half got the placebo. Researchers counted how many people got sick with COVID-19 after the vaccines reached their peak effectiveness.

Among both groups in the Pfizer trial, 170 people total got COVID-19, but only eight of those received the vaccine. That’s about 5% of total cases. That’s where the 95% estimate comes from. Moderna’s numbers were similar.

What does it mean that the COVID-19 vaccines also reduce severity?
More than 30,000 people total received the Moderna or Pfizer vaccines in both national research trials. Among those who did get COVID-19 after the peak effectiveness, there was one severe case. Just one.

Do the COVID-19 vaccines work against the new variants?
This is not completely clear because more research is needed. Pfizer announced that a preliminary study shows their vaccine is effective against the mutation in the spike protein in the two most common variants. And experts believe the Moderna vaccines will also be effective against the new coronavirus strains.

“Like with other vaccines, we don’t have just one antibody that forms, we have multiple and different antibodies,” said Dean Blumberg, chief of pediatric infectious diseases at UC Davis Children’s Hospital. “The antibodies attack the spike protein in several areas, so if one part changes, there are other areas we can have an immune response against.”

In addition, the vaccine makers say they can change the vaccine to adapt to new strains in as little as six weeks.

However, experts are still cautious. The variant that originated in South Africa has mutations that seem to help it hide from some antibodies, which could mean current vaccines would be less effective against it.

This is another reason why Blumberg and other experts urge people to get vaccinated as soon as it is available to them. Along with protecting their health, they will reduce the chances for the coronavirus to mutate by limiting the bodies it could grow in.

In contrast, more than 400,000 Americans have died from COVID-19 – more than one for every 1,000 Americans – at a rate of more than 3,000 people per day in January, according to the CDC tracker.

How well were the vaccines tested?

“The vaccines went through very large, very thorough studies with no shortcuts,” Blumberg said. “There were 43,651 people at more than 150 sites enrolled in the Pfizer/BioNTech clinical trial, including 225 at UC Davis Health. The results were carefully reviewed.”

Moderna’s trial included 30,351 people and went through the same exacting clinical trials and reviews.

“No safeguards were sacrificed,” Blumberg said.

Do the vaccines help everyone equally?

The findings from the clinical trials show that the COVID-19 vaccines are 95% effective for people across all subgroups. These include racial and ethnic minorities, people 65 years old and older, and those with one or more of these conditions:

  • Obesity
  • Diabetes
  • Hypertension
  • Chronic cardiopulmonary diseases.

One group that was not studied was children.

Could I get COVID-19 from the vaccine?

“There is absolutely no way you can get COVID-19 from the vaccine. It is not possible,” said Blumberg. “None of the vaccines currently authorized in the U.S. use the live virus. There is nothing in the vaccine that could cause COVID-19.”

How do the COVID-19 vaccines work?

“It really doesn’t matter which vaccine you receive. Both vaccines are very safe and about 95% effective. What does matter is that you get the vaccine.”
– Natascha Tuznik

The Pfizer and Moderna vaccines use mRNA, which stands for messenger ribonucleic acid. The mRNA, which is natural in our bodies, carries instructions to your cells. This mRA tells your body to make the spike protein that’s on the coronavirus.

Then your immune system recognizes the protein as foreign and develops antibodies to neutralize it. Your body remembers the protein and is ready to attack and eliminate the real SARS-CoV-2 virus.

“The vaccines teach your immune response to protect you from COVID-19,” Blumberg said. “The mRNA doesn’t stay around long. Your body breaks it down quickly and gets rid of it.”

Will I test positive for COVID-19 because of the vaccine?

“That can’t happen,” Blumberg said. “The virus is not in the vaccine. You can’t test positive on a PCR or an antigen test (the two methods that test for current COVID-19 infections).

“You will test positive for antibodies because your body will have built them up as part of your immune response,” he said. “That’s a good thing.”

Why do we need two shots with both COVID-19 vaccines?

After your first vaccination, your immune system begins to respond. It takes a few weeks to produce the T-lymphocytes and B-lymphocytes – the cells that mount your defense, produce antibodies and remember how to fight the virus.

The second shot creates a much greater immune response because you already have some cells ready to fight. It’s called the boosting phenomenon.

“After the second shot, your immune system really kicks into gear and multiplies those cells again many times over,” Blumberg said.

What are the differences between the COVID-19 vaccines?

“The vaccines are very similar,” Tuznik said. “The differences matter more to the people transporting and handling them.” Pfizer’s must be stored at -94 degrees Fahrenheit. Moderna’s needs -4 degrees Fahrenheit.

Both use messenger RNA to teach your immune system to neutralize the coronavirus.

Both are very safe and very effective and have similar temporary side effects – and those reactions are stronger after the second shot for both.

There are only two differences that impact the public:

  • Pfizer’s vaccine is authorized for people 16-years-old and up. Moderna’s is authorized for people age 18 and up.
  • The two Pfizer doses are given 21 days apart. The two Moderna doses are given 28 days apart.

“If I was given a choice,” Tuznik said, “I’d take the first vaccine offered.”

How were the COVID-19 vaccines developed so quickly?


The COVID-19 vaccines were extremely well-studied using the most demanding safety and efficacy standards.

  • The U.S Food & Drug Administration (FDA) and other experts reviewed the data from COVID-19 vaccine trials more quickly than usual by looking at data as it came in. In most cases, they wait until trials are complete. They used the same demanding safety and efficacy standards as always. No safety protocols were changed or skipped.

  • Vaccine manufacturers and the scientific community dropped everything to develop a vaccine. “There was a huge effort from universities, public health experts, manufacturers, epidemiologists and many others,” Blumberg said. “If you spend unlimited time and money, you can overcome a lot of problems really fast.”

  • The innovative mRNA approach has been in development since 2003. Researchers had already created the way of getting the mRNA into the body – what’s called an mRNA platform – for trials on cancer efforts and other vaccines. What they needed to learn was the genomic sequence of the coronavirus. “The vaccine platforms were developed just in case there was a pandemic,” Blumberg said. “Much of the research was to figure out what to put into the COVID-19 vaccine.”

“There is absolutely no way you can get COVID-19 from the vaccine. It is not possible. None of the vaccines currently authorized in the U.S. use the live virus. There is nothing in the vaccine that could cause COVID-19.”
– Dean Blumberg

Do I have to get a COVID-19 vaccine?

There is no government mandate to get a vaccine. But experts at UC Davis Health and health officials at every level are urging everyone to get the COVID-19 vaccine. The more people who get vaccinated, the faster the country, and the planet, can return to normal. Also, the more likely we can prevent a variant from developing that could evade the vaccines.

“We don’t anticipate any mandate,” Blumberg said. “It’s possible that might change. Of course, many businesses and schools may decide to require the COVID-19 vaccine, the way they do for influenza and other vaccines.”

Will I have a choice on which COVID-19 vaccine to get?

That’s unlikely for now. Once supplies ramp up, that may change.

“It really doesn’t matter which vaccine you receive,” Tuznik said. “Both vaccines are very safe and about 95% effective. What does matter is that you get the vaccine.”

What you need to know when you get the vaccine.

Read more about the vaccines from UC Davis Health experts.

Learn more about patient vaccinations at UC Davis Health.