COVID-19’s delta variant: What you need to know

Last updated July 30, 2021

Since the COVID-19 pandemic began, several variants of the virus have emerged. But the one named delta is now the most severe of them all. This highly contagious variant was first identified in India in December 2020. Within six months, it spread to 85 countries. Now it's in all 50 of the United States, where it's become the dominant strain of COVID-19.

Infectious disease expert David Tweardy, M.D., answers questions about this newest variant.

What is the delta coronavirus variant?

Originally known as B.1.617.2, the delta variant is the most infectious version of the COVID-19 virus to date. It was first identified in India in December 2020, where it quickly overwhelmed that country’s health care system.

How widespread is the delta variant in the U.S.?

It’s becoming more widespread every day. Infections nationwide tripled in July, from about 13,000 cases per day to more than 56,000. 

How contagious is this variant?

It’s extremely contagious, more than any other variant. Preliminary research suggests that the delta variant spreads from person to person more swiftly than the common cold or chickenpox.

SARS-CoV-2 is the original coronavirus that mushroomed out of Wuhan, China last year and led to the COVID-19 pandemic. Variants quickly emerged, including Alpha, which was first identified in the U.K. Alpha is estimated to be 50% more contagious than SARS-CoV-2. Delta is estimated to be 50% to 60% more contagious than Alpha.

What makes delta more contagious than other strains?

Delta infects people with a heavier virus load, which means they exhale more virus particles for others to catch. The delta variant’s mutations are also believed to make it more effective at attaching itself to cells in human airways.

Does it cause more severe illness than other variants?

Scientists are still investigating this, but a study published in The Lancet medical journal estimated that the risk of hospitalization after infection with delta could be 85% higher than with alpha.

In China, doctors are reporting that patients with the delta variant are sicker than those they treated early in the pandemic. And their condition seems to go downhill much faster. Recent Scottish studies suggest that people infected with the delta variant are almost twice as likely to be hospitalized as those infected with the alpha variant.

Does the Delta variant cause any unusual symptoms?

COVID-19 delta symptoms include headache, fever, sore throat – the same as those caused by other variants and the original SARS-CoV-2 virus. However, two typical COVID-19 symptoms are absent in delta patients – cough and loss of smell.

Who’s most at risk for the delta variant?

People who are unvaccinated.

In the United Kingdom, where delta is the dominant strain, studies showed that children and adults under age 50 are more than twice as likely to become infected. This is the age group with the lowest vaccination rate.

Will current COVID-19 vaccines work against the variant?

Yes, absolutely. We know that vaccines protect people against the delta variant. But a single dose doesn’t offer as much protection. It’s vitally important to follow up and get that second dose.

What about breakthrough infections?

It is possible that vaccinated people can still become infected with the delta variant, but those infections are infrequent. When they do occur, they're likely to be far less severe than infections in unvaccinted people. The vaccines have proven to be highly effective at preventing severe disease and death, but they are not 100% guaranteed to prevent you from getting the virus.

Can vaccinated people pass the delta variant to others?

New research suggests that vaccinated people who are infected with delta, even those who display no symptoms, may be able to transmit the virus as easily as those who are unvaccinated. Early studies show that infected vaccinated people have viral loads similar to infected unvaccinated people.

That's why the CDC has updated its mask-wearing guidelines and is now recommending that vaccinated people who live in high-transmission areas resume wearing maskes indoors.

Can we expect future delta mutations?

Viruses are built to mutate. With each mutation, a new and stronger variant can emerge. The best way to prevent this is to vaccinate as many people as possible. Variants are less likely to emerge when a pandemic is brought under control.

Do the COVID-19 vaccines protect immunocompromised cancer patients and survivors?

Yes, the COVID-19 vaccines are safe and recommended for cancer patients. Most people with cancer or a history of cancer are advised to get the COVID-19 vaccine. Some cancer treatments like chemotherapy, radiation, stem cell transplant, or immunotherapy can affect the immune system, which might make the vaccine less effective. People with cancers of the blood or lymph system can also have weakened immune systems which might lessen the vaccine’s effectiveness.

Despite this, we still recommend vaccination for cancer patients. Getting some protection from the vaccine is better than not having any protection. Every situation is different, so talk to your doctor about vaccine timing and the risks and benefits.

Young children can’t get vaccinated. What about them?

The FDA has not yet approved vaccines for children younger than 12 years. Children ages 12 and older are eligible for COVID-19 vaccines in the U.S. Parents should vaccinate their children as early as possible and, if they’re not eligible for vaccination yet, continue to take precautions, such as masking and social distancing. The Centers for Disease Control is now also recommending that all teachers, staff and students wear masks in schools, regardless of their vaccination status.

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