How Delta Became the Dominant Coronavirus Variant in the United States

Delta Coronavirus Variant

The first Delta case in the United States was diagnosed in March 2021 and has now become the dominant strain.

According to CDC data, the contagious delta variant (also known as B.1.617.2) has become the dominant form of the coronavirus in the U.S. which leaves many practitioners asking, “How did this happen?”

Viral Mutations

Viruses constantly change through mutating. According to the CDC, "a variant has one or more mutations that differentiate it from other variants in circulation. As expected, multiple variants of SARS-CoV-2 have been documented across the globe throughout the pandemic." The infamous Delta variant was first identified in India in December 2020 and has now been reported in over 95 countries including the Unites States.

Delta is More Contagious Than the Other Viral Strains

Research suggests that Delta is the most contagious of all the known SARS-CoV-2 variants to date. In fact, the World Health Organization (WHO) has called the Delta variant of the virus “the fastest and fittest.” This increased transmissibility has been seen in the United Kingdom where Delta now accounts for more than 95% of new COVID-19 cases.

Variant Classifications

A U.S. government SARS-CoV-2 Interagency Group (SIG) interagency group developed a Variant Classification scheme that defines three classes of SARS-CoV-2 variants:

  1. Variant of Interest
  2. Variant of Concern
  3. Variant of High Consequence

The B.1.1.7 (Alpha), B.1.351 (Beta), B.1.617.2 (Delta), and P.1 (Gamma), variants circulating in the United States are classified as “variants of concern” which are defined as:

A variant for which there is evidence of an increase in transmissibility, more severe disease (e.g., increased hospitalizations or deaths), significant reduction in neutralization by antibodies generated during previous infection or vaccination, reduced effectiveness of treatments or vaccines, or diagnostic detection failures.

(To date, no variants of high consequence have been identified in the United States.)

The Delta Variant Causes More Severe Illness

While more research is needed, there are indications that the Delta variant may cause more severe illness than other variants. A recent study published in The Lancet found that the risk of hospitalization from COVID-19 in Scotland was roughly doubled for patients infected with Delta compared with patients infected with the Alpha variant.

There also may be a difference in symptomatology for patients infected with the Delta variant. Based on hospitalizations in Britain, the most common symptoms of COVID-19 are now headache, sore throat, congestion, and fever while cough and anosmia (loss of smell) are less common.

Unvaccinated People are at High Risk

People who have not received the COVID-19 vaccine are most at risk for contracting the Delta variant. This explains why there is a COVID-19 surge in many states where vaccination rates are low. However, it is difficult to decipher if vaccinated people are not contracting the Delta variant at all or if they are contracting the virus but are simply not exhibiting symptoms. Regardless, it appears that vaccination does play a role in reducing hospitalization rates.

According to Yale Medicine, a pre-print Public Health England analysis proposed that the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease and 96% effective against hospitalization from Delta, while the Oxford-AstraZeneca was 60% effective against symptomatic disease and 93% effective against hospitalization.

Moderna and Johnson & Johnson have also reported that their vaccines have been proven to be effective in reducing hospitalizations from the Delta strain.

Children and Younger Adults are at High Risk

A U.K. study revealed that children and younger adults (under the age of 50) are 2.5 times more likely to become infected with the Delta variant. In fact, several countries have recently recorded a higher proportion of coronavirus cases among children.

Children are likely the most vulnerable population as few vaccines are currently available for those under the age of 16. Pfizer and Moderna expect to release trial data about the safety and efficacy of their vaccines in young children in the fall, then apply for FDA authorization. The good news is that children with the Delta variant are still at low risk for becoming severely ill.

What Patients Needs to Know

Since March 2020, healthcare professionals have had to be flexible and take a “learn as we go” mentality when it comes to coronavirus. The truth is there is still much to learn about the Delta variant and Delta Plus (a subvariant of Delta seen in several countries including the U.S.). We have been in and continue to be in an ongoing risk assessment phase. But from what we know today, vaccination is the best protection against Delta. Therefore, encourage your patients who can get the vaccine (no comorbidities preclude them from doing so), to get it as soon as possible.

For more vaccine information, click here.


  1. Center for Disease Control and Prevention (CDC): SARS-CoV-2 Variant Classifications and Definitions retrieved July 8, 2021.
  2. The Lancet 2021 June;397(10293):2461-2462.
  3. Yale Medicine: 5 Things To Know About the Delta Variant retrieved July 8, 2021.
  4. Gov.UK: Vaccines highly effective against B.1.617.2 variant after 2 doses retrieved July 8, 2021.
  5. Public Health England: Effectiveness of COVID-19 vaccines against hospital admission with the Delta (B.1.617.2) variant retrieved July 8, 2021.
  6. Imperial College London: REACT-1 round 12 report: resurgence of SARS-CoV-2 infections in England associated with increased frequency of the Delta variant retrieved July 8, 2021.
This article was written by Jami Cooley

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