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The Delta Variant Is Spreading In California. Immunized People Can Contract And Spread It. Here’s What You Can Do

People with masks walk at a farmer's market in North California.
People with masks walk at a farmer's market in North California.

The delta variant has changed California’s COVID-19 landscape in a big way.

New research from the U.S. Centers for Disease Control and Prevention shows that even fully immunized people can contract the virus and pass it onto others. But health experts say vaccines are still our best chance at reducing viral transmission since fully vaccinated people are far less likely to get infected.

Over the past few weeks, California has recommended masking indoors in public for all people, and more counties are making that mandatory, including Sacramento. Here's a look at some key questions around the delta variant, vaccines, and how to stay safe in this new stage of the pandemic.

A breakthrough case is when a person who is fully vaccinated tests positive for COVID-19. This happens because while the approved vaccines have been shown to be very effective at limiting the spread of the virus, they aren't 100% effective.

While there have been a rising number of breakthrough cases, they represent a small percentage of the total number of people who have been vaccinated. The number varies by state, but researchers estimate that less than 1% of fully vaccinated peoplehave gotten a COVID-19 diagnosis.

Why is the delta variant more transmissible?

Some research has shown that the delta variant results in a higher load of viral particles than the original strain that was dominant in California. This may be because of genetic mutationsthat makes this strain more capable of entering cells and suppressing immune function.

Viral load refers to the quantity of a virus in your body. It’s usually measured in terms of how much virus is in your blood or plasma. Scientists don’t know exactly what viral load is necessary to transmit the virus, but it’s likely a relatively small amount, which makes the disease more transmissible.

New research from the CDC suggests that people who are fully vaccinated can carry just as high a viral load as those who are not, though some researchers say it’s too soon to determine that. Positive cases of COVID-19 among vaccinated people are referred to as breakthrough cases.

“What we would think is that symptomatic breakthroughs, your immune system comes in, fights that virus, brings down that viral load sooner than someone who's unvaccinated,” said Dr. Monica Gandhi, an infectious disease specialist with the University of California, San Francisco. “So that's why I say I doubt that you're as infectious an unvaccinated person.”

Should I still get vaccinated if I haven't yet?


Even though both vaccinated and unvaccinated people can contract the delta variant, the odds of becoming severely ill and requiring hospitalization are much lower for those who are fully immunized. Most hospitalizations and deaths due to the delta variant have been among those whoare not fully vaccinated.

And Gandhi said that even in cases where a vaccinated person is infected, those milder symptoms mean they are infectious for a much shorter period of time.

“The promise of the vaccines is still there,” Gandhi said. “They are preventing severe disease and death. That promise is still there, that hope.”

When should you wear a mask now?

Sacramento and Yolo County health officials are requiring that everyone, regardless of vaccination status, wear a mask in public indoor settings. Los Angeles County and many Bay Area counties are also requiring masks indoors in public.

That means public transit, grocery stores, restaurants and other venues that are not checking vaccination status. Experts say if you’re vaccinated and you’re indoors with other vaccinated people, a mask isn’t necessary. If you’re going to be around people who are not vaccinated, it’s best to stick to outdoor interactions.

“The masks are back between the mix of the vaccinated and unvaccinated inside … certainly unvaccinated people should be masking,” Gandhi said. “Outside is really still considered safe.”

Gandhi says while it’s understandable that people might be upset about returning to masking, it’s a necessary and temporary precaution. Based on data from the UK, we could see a better picture in the United States around mid-September.

"I know [it] feels like a major setback, but this is the time to try to break that transmission, get those counts down and get more people vaccinated,' she said. "And then we will get through this."Should vaccinated people limit gatherings again?

Gandhi says it's OK for a small group of vaccinated people to gather. She said the new CDC recommendations are based on a very large group of people gathering from different areas in Provincetown, MA and that it's hard to generalize too much from that situation.

"So a typical inside small group of vaccinated people? Yes. You should please see your friends and family," she said.

Still, the Provincetown case did show vaccinated people can spread the virus when they're symptomatic. If you're heading for a large indoor gathering with vaccinated and unvaccinated people, you should be wearing a mask, and know that no social situation is 100% safe.

Would six-foot distancing around unimmunized people be considered necessary for an immunized person?

Yes, immunized people should keep their distance from unimmunized people when indoors as an additional precaution.

What should I do if I’m vaccinated and come into contact with someone who has a confirmed case of COVID-19?

If you were in close contact with someone who tested positive, meaning you were within six feet of someone for 15 minutes or more, you don’t have to quarantine but you should take precautions. This includes getting a COVID-19 test three to five days after exposure, and wearing a mask when indoors and in public for 14 days or until you get a negative test result. Many California counties are mandating that everyone wear a mask while in an indoor public space, whether they’ve had an exposure or not.

Fully vaccinated people do not need to quarantine after travel, according to current CDC guidelines.

How can breakthrough cases be a higher percentage of cases if the vaccine is working?

You may have seen reports from a recent CDC report of high rates of vaccinated people being infected in certain outbreaks or areas. One reason is that more people are being infected by the more virulent delta variant, but another is that there are more vaccinated people.

But if more people are vaccinated, shouldn't they make up a smaller portion of people infected? Yes and no. If there are many more people vaccinated in an area, even a small percentage of those people being infected can make up the majority of cases in an area.

Imagine a town with 100 people, 90 who are vaccinated and 10 who aren't. If a vaccine is 90% effective, around 9 of those people could get infected. If 90% of the 9 unvaccinated people are infected, then 8 unvaccinated people are infected. Now vaccinated people make up the majority of new cases, even though the vaccine is effective.

"And so as we get more and more people vaccinated, but they're still circulating virus, the person who's going to get a breakthrough is more likely to be the dominant group, which is the vaccinated group. But it doesn't mean that the immunity isn't working because our severe breakthrough infections among the vaccinated still remain low."Will vaccinated people need a booster shot?

This is unclear at the moment, but Gandhi said health officials are looking at boosters for some higher-risk patients with existing medical conditions.

"In terms of the severe disease among the vaccinated, we are absolutely seeing that, but it seems to be in the groups that are more immunocompromised, older patients with multiple medical conditions," she said. "And the CDC has been very clear that they are looking at giving boosters for that population."

For example, Israel has started to offer booster shots to people 60 and older, and San Francisco health officials are offering second shots, not boosters, to some people who had the Johnson & Johnson vaccine. But on Aug. 4, the World Health Organization asked wealthier nations to hold off on booster shots until more people around the world were vaccinated.

Sammy Caiola has been covering health care in California for the last seven years. Before joining CapRadio in 2017, Sammy was a health reporter at The Sacramento Bee. She has degrees in journalism and gender studies from Northwestern University.
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