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5 Things To Know About the Delta Variant

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As early summer brought a glimmer of hope – or at least cautious optimism – that the pandemic might ease into the background, the looming threat of new COVID-19 mutations remained. The possibility of an even stronger resurgence was always present.

Then came the Delta variant. First identified in India in December 2020, Delta spread rapidly through the country before reaching Great Britain and, ultimately, the United States. Once it arrived, it quickly became the dominant SARS-CoV-2 strain, responsible for over 99% of COVID-19 cases and driving surges in hospitalizations across several states.

Delta has proven to be more than twice as contagious as earlier variants, and studies suggest it may also lead to more severe illness. Those who remain unvaccinated are at the highest risk, with the most significant outbreaks and severe cases occurring in areas with low vaccination rates.

Dr. Inci Yildirim, a pediatric infectious diseases specialist and vaccinologist at Yale Medicine, wasn’t surprised by Delta’s rapid spread. “All viruses evolve and undergo changes as they spread and replicate,” she explains.

Current data indicates that fully vaccinated individuals still have strong protection against severe illness from COVID-19. However, concerns over Delta’s impact have led the Centers for Disease Control and Prevention (CDC) to recommend additional precautions, such as renewed mask guidelines for all individuals, regardless of vaccination status, and booster shots for added protection.

While the majority of Delta infections have occurred in unvaccinated individuals, the variant’s high transmissibility has also led to breakthrough cases among vaccinated people. According to the CDC, vaccinated individuals who develop symptomatic breakthrough infections can still spread the virus. Ongoing research is assessing whether asymptomatic vaccinated individuals can also transmit the virus.

Here’s what you need to know about the Delta variant.

1. Delta is more contagious than previous variants

The Centers for Disease Control and Prevention (CDC) has classified Delta as a "variant of concern," a designation also given to the Alpha variant first identified in Great Britain, the Beta variant from South Africa, and the Gamma variant from Brazil. The World Health Organization (WHO) introduced this naming system to replace the numerical designations previously used for variants.

Yale Medicine epidemiologist Dr. F. Perry Wilson notes Delta's rapid spread has been particularly striking. "Delta is spreading 50% faster than Alpha, which itself was already 50% more contagious than the original strain of SARS-CoV-2," he explains.

To illustrate this, Dr. Wilson provides a scenario: In an environment with no mitigation measures – where no one is vaccinated or wearing masks – the original coronavirus strain would likely be transmitted from one infected person to about 2.5 others. Delta could spread to 3.5 or even four other people in the same conditions.

"Because of exponential growth, even small increases in transmissibility can cause a virus to spread rapidly and dominate," he adds.

2. Unvaccinated individuals are most at risk

In the U.S., states with lower vaccination rates, including Alabama, Arkansas, Georgia, Mississippi, Missouri, and West Virginia, have experienced significant surges in Delta-related cases. Other regions have also seen rising infections. For example, in Idaho, one of the least-vaccinated states in the country, health officials were forced to ration medical care due to overwhelmed hospitals.

Children, teenagers, and young adults remain a concern. "A study from the United Kingdom found that children and adults under 50 were 2.5 times more likely to become infected with Delta than previous strains," says Dr. Inci Yildirim, a pediatric infectious diseases specialist at Yale Medicine.

While Pfizer-BioNTech applied for FDA authorization of its vaccine for children ages 5-11, no vaccine was approved for this group. As of late July, fewer than half of U.S. teenagers were vaccinated.

"As older populations achieve higher vaccination rates, younger unvaccinated individuals are at greater risk of infection," Dr. Yildirim notes. "Delta, in particular, appears to be affecting younger groups more than earlier variants."

3. Delta could trigger ‘hyperlocal outbreaks’

Dr. Wilson says if Delta continues to spread, its impact will likely vary depending on vaccination rates in different areas. "I call this ‘patchwork vaccination,’ where highly vaccinated areas exist alongside communities with only 20% vaccination coverage," he explains.

This creates conditions for the virus to "hop, skip, and jump" between low-vaccination areas, leading to localized outbreaks. "Some towns with low vaccination rates, surrounded by more protected areas, might see the virus remain contained within their borders," Dr. Wilson adds.

Experts debate whether the U.S.’s relatively high vaccination rate will help curb Delta’s spread. However, if the variant continues to drive up infections, it could lead to a steeper and faster pandemic curve.

"A compressed wave of infections might sound like it would end the pandemic sooner, but it's a serious problem," Dr. Wilson warns. "If too many people get sick at once, local healthcare systems will be overwhelmed, and preventable deaths will rise."

4. There’s still much to learn about Delta

Scientists are still studying Delta to determine its full impact. One key question is whether Delta causes more severe illness than previous variants.

Early studies from Scotland and Canada, cited by the CDC, suggested that unvaccinated individuals infected with Delta faced a higher risk of hospitalization. A study published in The Lancet Infectious Diseases found that people with Delta were twice as likely to be hospitalized in England than those with Alpha.

Another area of study is how Delta affects symptoms. "Reports suggest that symptoms such as cough and loss of smell are less common with Delta," says Dr. Yildirim. "Instead, people report headaches, sore throat, runny nose, and fever."

Scientists also monitor breakthrough infections, cases in which vaccinated individuals contract COVID-19. The CDC has stopped tracking non-severe breakthrough cases, making it difficult to determine exact numbers. However, it remains clear that no vaccine is 100% effective, meaning breakthrough cases are expected as case numbers rise.

Additionally, a subvariant known as Delta Plus has emerged in multiple countries, including the U.S. and the U.K. "Delta Plus carries an additional spike protein mutation, K417N also found in the Beta variant," Dr. Yildirim explains. More research is needed to determine whether Delta Plus spreads more easily or affects immunity differently.

5. Vaccination remains the best protection against Delta

Experts agree that complete vaccination is the most effective way to protect against Delta. For two-dose vaccines like Pfizer or Moderna, individuals must receive both shots and wait the recommended two-week period for complete immunity to develop. According to the CDC, vaccines protect against severe illness, hospitalization, and death.

To strengthen immunity, the CDC recommends booster doses of the Pfizer-BioNTech vaccine for:

  • People 65 and older
  • Residents of long-term care facilities
  • Individuals 18-64 with underlying health conditions
  • Those at increased risk due to their work environment, such as healthcare workers and teachers

A third dose is also available for people with weakened immune systems who may not have built sufficient immunity from the standard two-dose regimen.

Although breakthrough infections are possible, vaccinated individuals appear to carry the virus for a shorter period. It may reduce their likelihood of spreading it compared to unvaccinated individuals.

The CDC continues to recommend layered prevention strategies, such as wearing masks in indoor public spaces where transmission is high. Universal masking is also advised for students, teachers, and staff in K-12 schools, and local mask mandates may vary based on geography.

"As with any health risk, managing COVID-19 is about assessing and mitigating dangers," says Dr. Yildirim. "If you're outdoors in the sun, you wear sunscreen. If you're in a crowded indoor setting with unvaccinated individuals, you wear a mask. And if you're eligible, the best thing you can do is get vaccinated."

Despite the availability of vaccines, many people remain unvaccinated due to logistical challenges or personal choices. Whether Delta will be the push some need to reconsider remains uncertain.

"When local outbreaks occur, vaccination rates tend to rise," Dr. Wilson notes. "Seeing someone you know become seriously ill can change how you view the risk. I’m hopeful that we’ll see more people choosing vaccination."

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