COVID-19 and the Delta variant: What you need to know (video)


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The Delta variant of the COVID-19 virus has prompted new warnings from health officials and even new lockdowns in parts of the world. It’s now the dominant strain in the United States and California. Epidemiologists at UC Davis Health say the threat from this virus mutation highlights the importance of getting vaccinated.

Why is the Delta variant such a concern?


The Delta variant is highly transmissible, but epidemiologists say COVID-19 vaccines are effective against it.

The Delta variant is much more easily transmitted than other variants. “In the United States, it’s making up over 50% of our new cases,” said Lorena Garcia, professor of epidemiology in the Department of Public Health Sciences at the UC Davis School of Medicine. “In some states, it’s making up over 80% of new cases. Viruses are really smart. Their goal is to survive,” Garcia explained.  

Brad Pollock, chair of the Department of Public Health Sciences said this is typical. “These variants that happen after the original type of virus, they tend to make the virus more easily transmitted to somebody else,” he said. “The virus becomes much more efficient.”  

The risk in communities

Epidemiologists say with the Delta variant, individuals can shed the virus for longer periods of time, giving more opportunities for exposure. “With these variants, you’re at higher risk,” said Pollock. “You’re much more likely to get infected if you’re not vaccinated than you were six months ago.”

Lower numbers of vaccinated people mean the virus has more opportunities to spread and keep mutating.


UC Davis Health experts say the spread of the Delta variant has made COVID-19 vaccination critical.

“We do have pockets throughout the U.S. where individuals do not even have one dose of the vaccine, so there are communities that are at high risk. In particular, rural and semi-rural [communities] that do not have easy access to clinics and medical hospitals that have the resources to take on and treat the patients, these are the communities that we really need to worry about,” Garcia explained.

The Delta variant and vaccines

Breakthrough cases, when a fully vaccinated person becomes ill with COVID-19, are fairly rare. But Pollock noted none of the vaccines are 100% effective. “When the Pfizer and the Moderna vaccines were administered in the population, they were about 93% effective,” he said. “Which meant that for every 100 people you vaccinate, seven of them on average would end with a breakthrough case.”  Vaccine makers have said their vaccines have held up well against the Delta variant.  

If infection does occur after a person is fully vaccinated, the illness shouldn’t be severe. “If you got a full series of vaccinations, you’re very unlikely to be hospitalized or to die,” Pollock said. “In fact, many of these breakthrough infections may be asymptomatic, and the person may never know that they were infected.”  

What isn’t known is how easily vaccinated people can transmit the virus. “We have some ideas, that if you get infected after you’ve been vaccinated, you probably have lower amounts of virus that you’re going to shed,” Pollock said. “But unfortunately, we don’t have the population evidence yet that says with absolute certainty there’s no way you could transmit this to somebody else.”

“You’re much more likely to get infected if you’re not vaccinated than you were six months ago”

— Brad Pollock

Pollock pointed out that could become a problem in households with mixed vaccination status. “If you’re a breakthrough case and all your relatives at home aren’t vaccinated, theoretically you could infect them. That’s not good.”

Surge potential in California

Some states are seeing dramatic increases in infections due to the Delta variant in areas where vaccination rates are low. However, Pollock does not foresee lockdowns like we had in March, 2020, because of the presence of vaccines. “That has changed the game completely,” he said.

Sacramento County now has one of the highest COVID-19 infection rates in the state. The county’s vaccination rate is around 47%.

Preventive measures

Last month, the Centers for Disease Control updated its guidelines for people fully vaccinated against COVID-19, advising normal activities can resume, without wearing a mask or physically distancing, except where other rules apply.


UC Davis Health is offering COVID-19 vaccination for individuals 12 and older.

Shortly after, the rapid growth of the Delta variant prompted some health agencies to advise people, vaccinated or not, to wear masks indoors and outdoors when in a crowded area.  

For unvaccinated people, including children younger than age 12 who cannot get the vaccine, health experts strongly advise wearing masks indoors, when in groups or when social distancing is not possible. “If you’re unvaccinated, you need to be masked up,” Pollock said. Children under 2 should not wear masks, due to the risk of suffocation.

Both Pollock and Garcia urge anyone who can get the COVID-19 vaccine to do so. 

“The Delta variant has taught us that vaccination is even more important now. Being fully vaccinated really protects us and protects our community as well,” Garcia said.

Learn more about how to schedule your vaccination at UC Davis Health.

When to contact your provider

If you're experiencing COVID-19 symptoms, contact your primary care provider. UC Davis Health patients can use the MyUCDavisHealth symptom tracker. Telehealth video visits are also available during regular business hours.

If you have a medical emergency, call 911 and notify them of your COVID-19 symptoms.

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