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COVID-19 vaccines and variants: What you should know

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Dec 20, 2023.

Official answer


Which coronavirus variants are circulating in the U.S.?

The top 3 SARS-CoV-2 variants in the U.S. in early December 2023 are all of Omicron lineage and include:

  • HV.1 (30%)
  • JN.1 (21%)
  • EG.5 (9%)

Currently the JN.1 variant appears to be increasing, while HV.1 appears to be decreasing. These percentages are based on modeled predictions of variant proportions as reported by the US Centers for Disease Control and Prevention (CDC).

On Dec. 19, 2023 JN.1 was named a "variant of interest" by the WHO, which means health officials are now closely tracking its rapid spread across the globe. According to health authorities, it does not appear to cause more serious illness than previous variants. WHO states that the updated 2023-2024 COVID-19 vaccines are still expected to protect against infection from the JN.1 variant.

There are many different variants of the COVID-19 virus at any one time. In the U.S., sequence-based strain surveillance is ongoing in academic, state and public health government institutions. Other variants are currently circulating worldwide, and new variants may occur over time.

How dangerous are the coronavirus COVID-19 variants?

All viruses can mutate which leads to changes in their internal genetic code, creating "variants." Mutations are an expected part of the virus life-cycle, and not all variants are dangerous or harmful.

Scientists are researching how the new variants may alter COVID-19 disease, natural immunity, and the effect on vaccine effectiveness.

  • Some mutations of the coronavirus have been shown to be more contagious or to cause more serious COVID-19 illness.
  • Variants may affect the effectiveness of the COVID-19 vaccines. However, experts state that the virus would need to develop multiple mutations in the spike protein to evade immunity induced by vaccines or by natural infection.
  • A person's natural immunity from a previous COVID-19 infection could be affected by variants. This could lead to a spike in new cases. Variants theoretically may also alter the results of lab tests used to detect the virus or antibodies.

How do mutations affect vaccines?

Mutations can make vaccines hard to develop because of the change of the genetic footprint of the virus. Viruses often naturally mutate.

One example is the flu vaccine. New strains of influenza often appear worldwide each year, and those strains are incorporated into a new vaccine each flu season. Because the influenza vaccine gets a reboot each year, and it’s protective effect wanes over time, you need a yearly vaccine for protection against influenza.

But other viruses do not mutate so quickly. For example, the measles is slow to mutate. To protect yourself or your child from measles, one set of shots, typically given in childhood, is protective for a lifetime.

COVID-19 is a highly contagious viral infection due to the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) virus. The coronavirus is a long virus microscopically with 30,000 nucleotides and is twice the size of the flu virus. Coronavirus is an RNA virus, as are the flu and measles virus. Variants appear to have developed mutations in the spike protein of the coronavirus. The spike protein is the part that helps the virus to attach to infect human cells.

As of now, studies suggest that vaccination with the currently available vaccines in the U.S. recognize these SARS-CoV-2 variants. As the SARS-CoV-2 virus mutates over time, further vaccine updates may be needed, similar to how the influenza vaccine is updated yearly.

Which COVID-19 vaccines are available in the U.S.?

There are 3 COVID-19 vaccines currently recommended in the U.S:

Like any vaccine, these vaccines may not protect every person but can help prevent severe illness, hospitalization and death.

How do the COVID-19 vaccines work?


The messenger RNA (mRNA) vaccines encode a form of the spike (S) protein of SARS-CoV-2 virus. The vaccine teaches the cells to make a piece of the spike protein, which triggers an immune response to help prevent illness if later exposed to the virus. The partial piece of the spike protein cannot cause COVID-19 disease.

  • The dose for the Pfizer / BioNTech vaccine (for patients 16 years of age and older) is a series of two intramuscular (IM) injections in the upper arm given 3 weeks (21 days) apart.
  • The Moderna vaccine (for patients 18 years of age and older) is also a 2-dose series, but separated by 4 weeks (28 days). After administration, you’ll need to stay where you received the vaccine for 15 to 30 minutes for observation.

Commonly reported side effects with mRNA vaccines include injection site pain or swelling and flu-like symptoms such as fever, chills, tiredness, muscle and joint pain, and headache. These side effects subside in 24-48 hours in most patients. In some cases, side effects may be worse after the second dose than the first dose.

Severe allergic reactions (anaphylactic reactions) are rare but have been reported. These reactions can be treated, and most patients with allergies can still get the vaccine. It is recommended you do not use the vaccine if you have had a severe allergic reaction to any of the ingredients in the vaccine you receive, or to a previous dose of the vaccine.

Protein adjuvant

The 2023-2024 Novavax COVID-19 Vaccine, Adjuvanted contains 5 mcg of recombinant spike (rS) protein from the SARS-CoV-2 Omicron variant lineage XBB.1.5 and 50 mcg Matrix-M adjuvant. The Matrix-M adjuvant comes from saponins, naturally occurring compounds in the bark of the Quillaja saponaria (Soapbark) tree, and enhances the immune response to the vaccine.

The vaccine elicits an immune response to the rS protein, which protects against COVID-19.

How long does the COVID vaccine last?

COVID-19 is still spreading throughout the U.S., and is expected to continue. The duration of protection from the vaccines is not fully known yet, but Pfizer / BioNTech and Moderna have reported that immunity lasts at least 6 months with their vaccine. Pfizer has noted that their vaccine remains up to 91% effective with no safety concerns.

This is not all the information you need to know about COVID-19 and COVID-19 vaccines for safe and effective use and does not take the place of your doctor’s directions. Review the full product information and discuss this information and any questions you have with your doctor or other health care provider.


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