The most common adverse effects reported in the Phase 3 trials of the Moderna and Pfizer-BioNTech COVID-19 vaccines included injection site pain, fatigue, headache, muscle pain, chills and joint pain (Baden, February 2021; Polack, December 2020). The rates of local and systemic adverse events following vaccination were similar between the Moderna and Pfizer-BioNTech vaccines. These adverse events were more common in younger vaccine recipients (age <65 years in the Moderna trial, and age <55 years in the Pfizer-BioNTech trial) and after the second dose of vaccine. Similar findings were observed in an analysis of reports submitted to the Vaccine Adverse Events Reporting System and v-safe, which included data on 13,794,904 doses of mRNA COVID-19 vaccines administered in the U.S. from Dec. 14, 2020 to Jan. 13, 2021 (Gee, February 2021). 

The vaccine trains your immune system to recognize and defend your body from COVID, but it will not actually cause a COVID infection. The vaccine does not contain live COVID virus.

The vaccine is effective against COVID variants. A June 2021 study from the U.K. found that the Pfizer vaccine is 96% effective against hospitalization from the delta variant after two doses.

Getting infected with COVID does provide some natural immunity after your infection has run its course. However, this natural immunity decreases over time. Early studies indicate that the immunity provided by the COVID vaccine is longer-lasting than the immunity acquired through infection.

Viruses constantly change through mutation, and new variants of a virus are expected to occur. Sometimes new variants emerge and disappear. Other times, new variants persist. Numerous variants of the virus that causes COVID-19 are being tracked in the United States and globally during this pandemic. The best way to slow the emergence of new variants is to reduce the spread of infection by taking measures to protect yourself including getting a COVID-19 vaccine when available.

The mRNA in the Pfizer and Moderna vaccines never enters the nucleus of the cell where your DNA is located. The mRNA causes your cells to make protein to stimulate the immune system, and then the mRNA quickly breaks down — without interacting with your DNA at all.

The vaccine works. Many people are still getting infected in Missouri because around half of Missourians still have not received the COVID vaccine. In July 2021, around 97% of people hospitalized in the United States due to COVID symptoms were unvaccinated.

While unlikely, people who have been vaccinated can still get COVID. However, people who are vaccinated but become infected with COVID are much less likely to experience severe symptoms or require hospitalization.

The COVID vaccine will not affect fertility. The American College of Obstetricians and Gynecologists has stated that the COVID vaccine is safe for people who are thinking about becoming pregnant, wanting to become pregnant, are pregnant, or who are breastfeeding.

All vaccines and medicines do carry a small risk of adverse reactions. However, millions of people in the United States have safely received the COVID vaccine under some of the most intense safety monitoring in U.S. history.

The CDC and the FDA are currently monitoring reports that the Johnson & Johnson vaccine may have an increased risk of developing thrombocytopenia syndrome or Guillain-Barre Syndrome. It’s important to understand that these reported side effects are still incredibly rare.

It’s also important to note that reports of adverse events to the Vaccine Adverse Event Reporting System (VAERS) following vaccination, including deaths, do not necessarily mean that a vaccine caused a health problem. It simply means that a patient experienced an adverse health event within a certain timeframe after receiving a vaccine. Correlation does not always equal causation.

These reports are rare, given the hundreds of millions of vaccine doses administered. They have been reported after mRNA COVID-19 vaccination. Confirmed cases have occurred mostly in male adolescents and young adults aged 16 years or older, are seen more often after getting the second dose than after the first dose of one of the two mRNA COVID-19 vaccines, and typically occur within several days after COVID-19 vaccination. CDC continues to recommend COVID-19 vaccination for everyone 12 years of age and older, given the risk of COVID-19 illness and related, possibly severe complications, such as long-term health problems, hospitalization, and even death.

Thrombosis with thrombocytopenia syndrome (TTS) is a rare, clinically serious and potentially life-threatening condition. Current evidence suggests a plausible causal association with the Janssen COVID-19 Vaccine. Most cases are in women, with most aged 18-49 years old. TTS does not appear to be associated with mRNA COVID-19 vaccines.

Pfizer-BioNTech vaccine

Type: mRNA vaccine*

Approved for Ages: 12 years and older

Dosing: 2 doses 21 days or more days apart

Booster: third dose boosters are available for qualifying patients

Effectiveness: 91% effective at preventing the coronavirus disease 2019 (COVID-19) virus with symptoms in people age 16 and older

100% effective at preventing the COVID-19 virus in children ages 12 through 15

Appears to protect against severe COVID-19

Moderna vaccine

Type: mRNA vaccine*

Approved for Ages: 18 years and older

Dosing: 2 doses 28 days or more apart

Booster: third dose boosters are available for qualifying patients

Effectiveness: 94% effective at preventing the COVID-19 virus with symptoms

Appears to protect against severe COVID-19

Janssen/Johnson & Johnson vaccine

Type: Vector vaccine*

Approved for Ages: 18 years and older

Dosing: one dose

Booster: no second or third dose is needed

Effectiveness: 66% effective at preventing the COVID-19 virus with symptoms

Messenger RNA (mRNA) vaccine. This type of vaccine uses genetically engineered mRNA to give your cells instructions for how to make the S protein found on the surface of the COVID-19 virus. After vaccination, your immune cells begin making the S protein pieces and displaying them on cell surfaces. This causes your body to create antibodies. If you later become infected with the COVID-19 virus, these antibodies will fight the virus.

After delivering instructions, the mRNA is immediately broken down. It never enters the nucleus of your cells, where your DNA is kept. Both the Pfizer-BioNTech and the Moderna COVID-19 vaccines use mRNA.

Vector vaccine. In this type of vaccine, genetic material from the COVID-19 virus is placed in a modified version of a different virus (viral vector). When the viral vector gets into your cells, it delivers genetic material from the COVID-19 virus that gives your cells instructions to make copies of the S protein. Once your cells display the S proteins on their surfaces, your immune system responds by creating antibodies and defensive white blood cells. If you later become infected with the COVID-19 virus, the antibodies will fight the virus.

Viral vector vaccines can’t cause you to become infected with the COVID-19 virus or the viral vector virus. Also, the genetic material that’s delivered doesn’t become part of your DNA. The Janssen/Johnson & Johnson COVID-19 vaccine is a vector vaccine. AstraZeneca and the University of Oxford also have a vector COVID-19 vaccine.

Vaccine efficacy refers to the percent reduction in cases of a disease among individuals who receive a vaccine compared with those who are unvaccinated. The primary efficacy endpoint in all the trials was clinical disease, meaning symptomatic COVID-19; reduction in infection, which would include both symptomatic COVID-19 as well as any positive test for SARS-CoV-2 in the absence of symptoms, was not assessed as a primary endpoint, although additional data utilizing serologic endpoints are being collected in all the trials. When the term “vaccine efficacy” is discussed in relation to these vaccines, it generally refers to efficacy at preventing clinical disease unless otherwise specified. 

The primary endpoints for the Phase 3 trials of these vaccines were as follows: 

• Pfizer-BioNTech: Efficacy against PCR-confirmed symptomatic COVID-19 with onset at least 7 days after the second dose of vaccine among participants without serologic or virologic evidence of prior SARS-CoV-2 infection at baseline. 
• Moderna: Efficacy against PCR-confirmed symptomatic COVID-19 with onset at least 14 days after the second dose of vaccine among participants without evidence of prior SARS-CoV-2 infection at baseline. 
• Johnson & Johnson/Janssen: Efficacy against PCR-confirmed moderate to severe/critical COVID-19 in the periods starting 14 days after vaccination and 28 days after vaccination among participants without evidence of prior SARS-CoV-2 infection at baseline.