COVID-19 Vaccine

Updated Aug. 25, 2021

Like many other viruses, SARS-CoV-2 — the virus that causes COVID-19 — changes over time. Those changes, or mutations, can lead to new variants of the virus.

There are many variants of SARS-CoV-2 now spreading globally. Scientists are studying them to see how they differ from each other and from the original strain of SARS-CoV-2.

Scientists also are monitoring how well authorized vaccines and therapies that can prevent or treat COVID-19 are working against the variants.

“Because there has been so much transmission globally, it means the virus has had an opportunity to diversify,” says Graham Snyder, MD, medical director, Infection Prevention and Hospital Epidemiology, UPMC. “And when the virus diversifies — when it looks different — that means our immune systems may be less prepared to recognize it.”

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How Many COVID-19 Variants Are There?

Virus mutations are random. It is not unusual for viruses to change over time, Dr. Snyder says. That’s what happened with SARS-CoV-2.

“Every single day, there are hundreds of thousands of transmission events around the world,” Dr. Snyder says. “And when you have hundreds of thousands of transmission events a day, that’s a huge opportunity for something different to emerge.”

There are many variants of the coronavirus spreading globally and within the United States. The Centers for Disease Control and Prevention (CDC) classifies variants into three groups:

  • Variants of interest (VOI): Variants that potentially may cause higher transmission, more severe illness, or may affect existing therapies or vaccines. Variants classified as VOIs may have caused clusters of cases. However, few of these variants have appeared in the U.S. or other countries.
  • Variants of concern (VOC): Variants that have shown some evidence of causing more severe illness or higher transmission rates, or that have shown some evidence of affecting existing vaccines or therapies.
  • Variants of high consequence: Variants that have shown clear evidence of impacting the effectiveness of vaccines or therapies. They also have shown evidence of causing more severe illness and increased hospitalizations.

There are currently no variants of high consequence in the U.S. The CDC is currently tracking several variants of interest or variants of concern.

The most widely known variant of concern is the Delta variant (B.1.617.2). It has become the most widely circulating variant of SARS-CoV-2 in the U.S.

The Delta variant is the most contagious variant of SARS-CoV-2 yet. According to the CDC, it is more than twice as contagious as previous variants.

Some data report that Delta also causes more severe illnesses than other variants, but more data is needed.

Scientists continue to study all variants. Their full effect on transmissibility and severity of illness is not yet known. It’s possible more variants will emerge in the future.

“These emerging strains are likely to be more transmissible,” Dr. Snyder says. “They may be more likely to cause severe disease. For the most part, the strains that we’re seeing, we’re still well prepared to respond to them.”

Does the COVID-19 Vaccine Work Against Variants?

There are currently three COVID-19 vaccines being distributed in the United States: from Pfizer-BioNTech, Moderna, and Johnson & Johnson/Janssen (J&J). All three vaccines are safe and effective in preventing COVID-19.

Available evidence indicates the vaccines are effective against COVID-19 variants as well, including the Delta variant. They are especially effective in preventing severe illness, including hospitalization and death from COVID-19.

“If you’re worried about emerging variants, the most important thing you can do is not give them a chance to transmit,” Dr. Snyder says. “It’s still wise to get the vaccines because they work against what we’re likely to see in the U.S. in the future.”

Scientists will continue to monitor how effective the vaccines are against variants.

In August 2021, federal health officials announced a plan to provide a booster dose of the Pfizer and Moderna vaccines to people who received the first two doses of those vaccines. Immunocompromised patients — like organ transplant recipients — also are eligible for booster doses of the Pfizer and Moderna vaccines.

There is no current plan for a J&J booster, but a booster could become necessary for that vaccine as well.

Do Monoclonal Antibodies Work Against COVID-19 Variants?

Monoclonal antibodies — which act like your body’s natural antibodies but are created by scientists — are a promising treatment for COVID-19.

So far, the Food and Drug Administration has authorized multiple monoclonal antibody therapies to treat COVID-19:

  • The drugs casirivimab and imdevimab (REGEN-COV) used in combination.
  • The drug sotrovimab.

Monoclonal antibodies have shown great success in preventing severe illness, hospitalization, and death if given shortly after infection. So far, reported data say they have also been effective against COVID-19 variants, Dr. Snyder says.

Scientists continue to monitor current COVID-19 variants and will monitor others that emerge. As they learn more information, it could affect guidelines for COVID-19 vaccination and/or treatment.

Visit to learn more about UPMC’s response to COVID-19. Go to to schedule a COVID-19 vaccine appointment with UPMC if you are eligible to receive one.


Centers for Disease Control and Prevention, About Variants of the Virus that Causes COVID-19. Link

Centers for Disease Control and Prevention, SARS-CoV-2 Variant Classifications and Definitions. Link

Centers for Disease Control and Prevention, COVID-19 Data Tracker, Variant Proportions. Link

Dan Frampton, PhD, et al, The Lancet: Infectious Diseases, Genomic Characteristics and Clinical Effect of the Emergent SARS-CoV-2 B.1.1.7 Lineage in London, UK: A Whole-Genome Sequencing and Hospital-Based Cohort Study. Link

Moderna, Moderna COVID-19 Vaccine Retains Neutralizing Activity Against Emerging Variants First Identified in the U.K. and the Republic of South Africa. Link

Moderna, Moderna Announces Positive Initial Booster Data Against SARS-CoV-2 Variants of Concern. Link

New England Journal of Medicine, Effectiveness of the BNT162b2 Covid-19 Vaccine against the B.1.1.7 and B.1.351 Variants. Link

Jerald Sadoff, MD, et al, New England Journal of Medicine, Safety and Efficacy of Single-Dose Ad26.COV2.S Vaccine against Covid-19. Link

About UPMC

Headquartered in Pittsburgh, UPMC is a world-renowned health care provider and insurer. We operate 40 hospitals and 700 doctors’ offices and outpatient centers, with locations in central and western Pennsylvania, Maryland, New York, and internationally. We employ 4,900 physicians, and we are leaders in clinical care, groundbreaking research, and treatment breakthroughs. U.S. News & World Report consistently ranks UPMC Presbyterian Shadyside as one of the nation’s best hospitals in many specialties and ranks UPMC Children’s Hospital of Pittsburgh on its Honor Roll of America’s Best Children’s Hospitals.