The delta variant is a strain of SARS-CoV-2, the virus that causes COVID-19. According to the Centers for Disease Control and Prevention (CDC), the delta variant was first identified in India in December 2020, and it was detected in the United States in March 2021. By the end of July, it became the dominant variant in the U.S. More than 80% of new COVID cases are attributed to the delta variant, and severe cases are primarily among unvaccinated people. 

The Delta Variant Is More Contagious

The delta variant is highly transmissible compared to earlier COVID variants. A recent study out of the United Kingdom found a 64% increase in the odds of transmission associated with the delta variant compared to the alpha variant, the previously dominant COVID variant.

Viruses are constantly changing, and new variants will differ in terms of how quickly a virus will spread, how sick people will get from it and how resistant it will be to treatment or vaccines. New variants of viruses occur when a virus attaches to a new host cell and undergoes genetic mutations as it replicates.

“Often, mutations are of little consequence to the behavior of the virus, but sometimes a mutation will cause it to become more ‘fit,’” said Joyce Sanchez, MD, Froedtert & MCW infectious disease specialist and director of the Froedtert & MCW Travel Health Clinic. “A mutation can make the virus more contagious, allow it to replicate more readily in our cells or survive in the environment for longer periods of time. The delta variant is one strain that has become more fit by being more transmissible than the ancestral strain.”

Delta Variant Symptoms Are the Same as Earlier COVID Variants

COVID symptoms associated with the delta variant have not changed in a considerable way since the original strain of the SARS-CoV-2 virus. The virus presents with a wide range of flu-like symptoms.

The most common COVID symptoms are:

  • Fever
  • Cough
  • Shortness of breath
  • Fatigue
  • Muscle aches
  • Headache
  • Sore throat
  • Congestion or runny nose

“Loss of taste and smell is another common and distinguishing feature, though some studies suggest there may be less cough and loss of taste and smell with the delta variant compared to the ancestral strain,” Dr. Sanchez said. “Also, some patients may have infection and be asymptomatic; in other words, they do not present with any symptoms.”

Unvaccinated People Are Most at Risk

Anyone who has not recently recovered from COVID-19 or been fully vaccinated is more at risk for developing infection from the delta variant, particularly in communities where transmission rates are high and when public health precautions are not taken. Risk factors for severe disease, hospitalization and death include older age (ages 65 and over), obesity and medical problems such as diabetes, heart disease, lung disease or immunosuppression.

In August 2021, vaccine data from the Wisconsin Department of Health Services (DHS) showed just over half of Wisconsinites were fully vaccinated against COVID-19. According to DHS, more than 98% of COVID cases in Wisconsin between January 2021 and the end of July 2021 were among people who were not fully vaccinated.

COVID Vaccines Are Effective Against the Delta Variant

Getting vaccinated is the best way to protect yourself and your loved ones from COVID-19. According to the CDC, all three COVID vaccines offer protection from the delta variant. The Johnson & Johnson vaccine is one dose, and the Pfizer-BioNTech and Moderna vaccines are both a series of two doses. The CDC recommends that people whose immune systems are compromised receive a third dose (booster) of the Pfizer-BioNTech and Moderna vaccines. The Food and Drug Administration (FDA) granted full approval to the Pfizer-BioNTech vaccine in August 2021 for individuals ages 16 and over. The Moderna and Johnson & Johnson vaccines are approved by the FDA for emergency use.

The Pfizer-BioNTech and Moderna vaccines are messenger RNA vaccines — also known as mRNA vaccines — that deliver a small piece of genetic code from the virus that causes COVID-19 to infect your cells, showing your body how to mount an immune response and produce antibodies against it. The Johnson & Johnson vaccine uses an adenovirus, a common virus that causes cold-like symptoms, containing information about the virus that causes COVID-19 to instruct your immune system on how to protect against a COVID infection.

According to the FDA, the agency responsible for evaluating the safety and effectiveness of the COVID vaccines, people who have been vaccinated are at a significantly lower risk of developing infection, including mild or severe disease and death due to COVID-19. This is because a fully vaccinated person’s immune system is primed to recognize the virus and kill it before it has time to replicate and cause disease.

“The original studies from 2020 showed that the Moderna and Pfizer-BioNTech vaccines were more than 90% effective against infection and nearly 100% effective in preventing hospitalization and death. Even with the more contagious delta variant, to date, the available vaccines have demonstrated to still be very effective in preventing infection, especially for severe disease and death,” Dr. Sanchez said.

Vaccine manufacturers continue to investigate whether those who have been fully vaccinated have measurable immune responses against variants as they arise. The CDC cites a study published in July 2021 in the New England Journal of Medicine that looked at the effectiveness of the Pfizer-BioNTech and Moderna vaccines against the delta variant. Researchers found only a modest difference in effectiveness compared to the original alpha variant.

Breakthrough Infections Are Usually Asymptomatic

A breakthrough infection is a term used to describe an infection that occurs in a person who has been fully vaccinated against COVID-19. While the COVID vaccines are highly effective, no vaccine is 100% effective at delivering immunity.

“Breakthrough infections are generally asymptomatic or symptoms are mild,” Dr. Sanchez said. “These cases are inevitable when people are living in communities with high rates of transmission and where a large proportion of people are not immune, either through vaccination or recent recovery from infection."

Breakthrough infections are usually detected when screenings take place for travel, sports or employment purposes. Most people who experience a breakthrough infection report mild or no symptoms. Cases of severe disease, hospitalization and death in fully vaccinated individuals are extremely rare, and the risk is dramatically lower than in those who have not been fully vaccinated.

It’s Rare, But Vaccinated People Can Spread COVID-19

The small proportion of fully vaccinated people who experience a delta variant breakthrough infection can spread the virus to others, according to the CDC. This became clear after CDC outbreak investigations, including the July 2021 outbreak in Massachusetts. The CDC recommends everyone wear a mask indoors in public settings in areas of substantial or high transmission, regardless of vaccination status. This map shows the level of community transmission in your area.

The CDC says there is an increased amount of virus, also known as viral load, produced by the delta variant compared with earlier COVID variants. Preliminary research indicates both unvaccinated and fully vaccinated people produce the same amount of virus once infected. However, fully vaccinated people are likely infectious for less time.

COVID-19 Can Have Long-Term Effects

People who had COVID-19 and recovered can experience health problems months later. Organ damage (particularly to the lungs, heart and brain) and autoimmune conditions are post-COVID conditions that people who survived severe COVID infections may experience. But, even young, healthy individuals who experienced mild disease or people who had minimal or no symptoms while they were infected can develop more long-term effects, known as Post-Acute Sequelae of COVID (PASC), also called “long-COVID,” “long-haul COVID,” “chronic COVID” or “post-acute COVID-19.”

“Several symptoms, including chronic fatigue, memory problems and brain fog can last weeks to months and interfere with a person’s ability to live as they did previously,” Dr. Sanchez said.

Post-COVID conditions typically present four or more weeks after infection. The CDC and other infectious disease experts are studying post-COVID conditions to learn more about who is at risk and why.

Froedtert & MCW patients experiencing COVID symptoms eight or more weeks after diagnosis are eligible for treatment through the Froedtert & MCW Post-COVID Multispecialty Clinic. Interested patients should discuss their symptoms with their Froedtert & MCW provider.

The Bottom Line: Vaccination Is the Best Protection and Prevention Against the Delta Variant

Being fully vaccinated:

  • Protects you from the virus
  • Lessens severity of disease if infected
  • Protects those who are not yet eligible to get vaccinated (children under the age of 12)
  • Protects those who may not be able to mount a robust immune response to the vaccines (immunocompromised people)
  • Prevents the virus from replicating and potentially mutating, leading to more variants

There is still a lot to learn about the delta variant, including if it causes more severe disease in nonimmune individuals, including young, healthy people. At this time, the data is mixed. Although uncommon, researchers are also studying how often breakthrough infections in fully vaccinated people occur and how fully vaccinated people can spread COVID-19 to others. Research is also underway regarding booster vaccines to determine if they will be needed in the future due to new variants, waning immunity or both and whether certain individuals (such as the elderly or immunocompromised) may develop waning immunity faster than others.

For more COVID vaccine information and updates, visit:

advance helthcare

I have honestly never read such overwhelmingly good content like this. I agree with your points and your ideas. This info is really great. Thanks.

Barbara Hawk

Thank you for providing this information.



Karen K-H

Great and very informative information! Thank you!

Lloyd Sartin

Thank you for this informative piece. I wish we could do more for the younger generation.

Julie B

I would like to know the data on percentage of unvaccinated people getting Covid vs. Unvaccinated but have already had Covid and have natural immunity. Are the Covid patients mainly unvaccinated that never had Covid yet? There should be numbers comparing all the pertinent data.

Vaccinated - but still got Covid
Unvaccinated - got Covid
Unvaccinated - but had Covid previously
This would also be helpful information. Knowledge is power.