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COVID-19 Variants & Vaccines: Why Does a Virus Mutate?

This information was reviewed and approved by Jared J. Eddy, MD (December 30, 2021).

As the virus continues to evolve with the Delta and Omicron variants, many are asking how effective are the vaccines against the mutant or variant strains of the SARS-CoV-2 virus.

vaccine variants

Why does the virus mutate?

Viruses, like all living things, go through mutations throughout their lifespan. If a mutation provides a survival advantage, then that mutated version of the virus starts to outcompete with other versions. With enough of the “right” mutations it may become the dominant variant in a human population. Some mutations make the virus more transmissible and contagious while others help it to evade the immune system. Just like the chameleons have evolved to change skin pigment shades to avoid predators, the COVID-19 virus keeps on evolving to avoid its “predator” – the human immune system.

Viruses constantly mutate but with enormous amounts of virus in large numbers of people, the human population is a giant laboratory for developing many different mutations and combinations of mutations. Ultimately, fewer unvaccinated and infected people mean fewer problematic variants.


Do mutations make big changes in a virus?

A mutation is a change in the genes of the virus. Some mutations make the virus more unstable or less effective. Other mutations may increase rates of transmission or disease severity. We become concerned when mutations make the virus 1) more contagious, 2) less able to be managed by our immune system or treatments/vaccines, and 3) more “virulent” which means causing more severe disease.


How many variants of COVID-19 are there?

The number of variants is ever-changing but in general there are usually a handful of important variants to keep track of at any time. Institutions such as the World Health Organization (WHO) and The Centers for Disease Control and Prevention (CDC) categorize variants based on levels reflecting how likely they are to negatively impact the population and the strength of evidence that they are doing so (that is to say are they circulating widely, evading therapies, or causing more severe disease). Currently, the CDC lists 10 as Variants Being Monitored, zero as Variants of Interest, two as Variants of Concern and zero as Variants of High Consequence.


How are variants named?

Each variant is given a scientific name and then the World Health Organization (WHO) names COVID-19 variants with letters of the Greek alphabet to help prevent public confusion and stigma against the places they were first identified. The WHO website lists five variants of concern around the globe: Alpha, Beta, Gamma, Delta and Omicron.


What are the variants of concern in the U.S. right now?

The Delta variant, B.1.617.2, which is more transmissible, has been blamed for India’s explosive outbreak, and has become dominant in the U.S. where Alpha was once the most common variant.

On November 26, 2021, the World Health Organization (WHO) classified a new variant, B.1.1.529, as a Variant of Concern and has named it Omicron.


What is the Delta variant?

The Delta variant was first identified in India in December 2020 and has been blamed for India’s explosive outbreak. Delta arrived in the U.S. and became the dominant of all the current variants. It is more than twice as contagious and transmissible than previous variants, and accounts for more than 99% of current cases in the U.S. Vaccines in the U.S. are highly effective against the Delta variant. Evidence indicates that only a small number of fully vaccinated people get the Delta variant, and they transmit it for a shorter time than unvaccinated. Being unvaccinated increases your risk of being infected with this variant. The vast majority of COVID hospitalizations and deaths are in the unvaccinated.


What is the Omicron variant?

The Omicron variant was first identified in South Africa in early November 2021. Initial cases were identified in Canada, Belgium, Botswana, Hong Kong, Australia, Scotland, Portugal and Israel and the situation continues to evolve. This variant shares many key mutations with other variants of concern, but has many new mutations in the part of the virus that infects human cells. This does not necessarily make it more deadly. Scientists are still learning if Omicron is more transmissible than Delta and if it will cause more severe disease compared to Delta. The fact that it is spreading so quickly means it is likely more transmissible but anecdotes suggest it might cause milder disease (we hope!).

Currently it looks like the symptoms are the same as other variants - fever, chills, cough, shortness of breath, fatigue, muscle or body aches, headache, sore throat, loss of taste or smell, and sore, congested and runny nose. Early data suggests that people who had COVID-19 previously may have an increased risk of getting infected again with Omicron.

Despite the many unknowns about this variant, getting vaccinated, wearing masks and washing hands frequently still offer the best protection against getting sick.

Watch the Centers for Disease Control and Prevention for updates on variants.


What are the differences between the Delta and Omicron variants?

(Quickly Evolving Information)
TransmittedMore transmissible than the common cold, flu, smallpox, MERS, SARS, Ebola
Unknown but being closely watched
Likely very transmissible
SpreadSpreads in respiratory droplets from coughs and sneezes as well as short-range airborne aerosols
Key MutationsAbility to infect more cells and increase the amount of viral RNA that makes it into host cellsShares mutations with previous variants that may make them less vulnerable to vaccines
Has unique mutations in areas targeted by vaccine antibodies
Viral LoadPeople carry 1,000X Delta variant in their noses compared to original virus
Exposure to Infection4 days on averageUnknown, but likely similar to other variants
Difference in Symptoms 
from Original Virus  
Fever, headache, sore throat, runny nose
Can also include hearing and stomach issues, and blood clots
Less common are cough and loss of smell
Cough, fever, night sweats, body pain, fatigue, headache, sore/scratchy throat
ResultsCaused spike in hospitalizations in the U.S. over a few weeksHospitalizations in South Africa tripled in 14 days
ProtectionsVaccination prevents serious disease and death for all variants and is likely to do so also for Omicron


Do COVID-19 tests tell me which variant I have?

According to the Food and Drug Administration (FDA) there are no authorized antigen or molecular COVID-19 tests that report the present of a specific variant. These tests check for the original SARS-CoV-2 virus. Variant-specific tests may be available in the future.


How long are people contagious with the Delta or Omicron variants?

With the original COVID-19 you could be contagious 48 hours before having symptoms. People who contract the Delta variant of COVID-19 are usually contagious for about 10 days after symptoms appear in mild cases, and up to 20 days in moderate to severe cases. It is not yet known how long people are contagious with the Omicron variant.


Is the treatment different for Delta or Omicron?

Treatment at home for mild cases still targets relieving symptoms so your immune system can fight the infection. Home treatments can include fever reducing medications, hydration and rest. If you have risk factors for severe COVID-19, monoclonal antibodies given within 10 days can help prevent progression to severe disease. Hospital care for more serious cases of COVID-19 can include antiviral medications, treatment to reduce an overactive immune response to the virus, blood thinners to reduce blood clots, and other treatments depending on other health issues. As we learn more about Delta and Omicron, new treatments will be evaluated.


What is the recovery time for people with the Delta or Omicron variant?

Most healthy people will need at least two weeks to recover from a mild case of the Delta variant of COVID-19. More serious infections can take six weeks or more for recovery. It is not yet known how long it will take to recover from an Omicron infection.


How can I protect myself from these variants?

Staying up to date with COVID-19 vaccination including a booster will remain a key measure to help prevent people from becoming infected and prevent a serious outcome if they are infected. Even if the current vaccines turn out to perform quite poorly against Omicron, new versions of vaccine targeting Omicron are currently being made and would soon be brought to market. Other pandemic precautions will help prevent infection regardless of which variant you are exposed to: wearing face masks, physical distancing, avoiding poorly ventilated areas, and hand washing. A combination of overlapping measures provides the best protection.


Are these strains more deadly than SARS-CoV-2?

So far, it does not look like that there is any difference in the disease severity caused by these strains vs. the original SARS-CoV-2 virus and there is anecdotal evidence that Omicron may cause milder disease. Evidence suggests that the strains are no more deadly than the original virus. However, variants like Omicron may be more difficult for the immune system of a person who has had COVID or has been vaccinated to neutralize, and monoclonal antibodies may be less effective.


Do the vaccines protect us against these new variants?

So far, the studies have shown that the mutations found in variants have not changed the “big picture” of the virus. The immune system can still recognize the virus and act against it even if there are mutations, albeit with slightly less efficacy for some variants.

In the last month multiple studies have shown that, all available vaccines can neutralize variants in the lab. Real-life data show that the vaccines, even with the variants, provide a very high degree of protection against mild or moderate disease, and dramatically reduce the risk of severe disease and death even. Studies are ongoing to prove that this will also be true for Omicron.

Bottom Line:

The vaccines work well against the virus and variants. New strains can be stopped by widespread vaccination. In the meantime, we should all:

  • Get the vaccine, including recommended boosters, as soon as possible
  • Still wear a mask indoors for now
  • Limit large indoor gatherings with poor ventilation
  • Keep washing our hands
  • Protect the vulnerable



The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. and your state’s health department may offer additional guidance.

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