However, health officials say the vaccine is still the best defense against the virus.
The highly contagious variant of the novel coronavirus that was initially identified in India is now the dominant strain in the United States, according to the Centers for Disease Control and Prevention.
Health officials and experts alike have warned that the delta variant is a more infectious version of the disease, and preliminary data indicates it may increase the risk of hospitalization. The variant has also shown to be particularly dangerous to those who are unvaccinated or partially vaccinated against COVID-19.
However, current evidence suggests that the full dosage of a COVID-19 vaccine is highly effective in preventing hospitalizations and serious illness.
Data updated by the CDC last week shows the so-called delta variant, also known as B.1.617.2, was estimated to account for 51.7% of all new cases of COVID-19 across the country as of July 3.
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In June, after more than a year of pandemic stress, Sean Fruit and his wife decided to go to Las Vegas.
They were fully vaccinated and drove to their destination to avoid airplanes as an extra precaution.
Once there, Fruit said they did not wear masks.
"I took the option to take my mask off, and now that I look back on it, probably a bad idea," said Fruit.
When they got back home to their two children in San Benito County, Fruit started to feel sick. He and his wife tested positive for COVID-19.
"It was extremely frustrating because we just spent a year and a half avoiding this thing and it was scary. I mean I brought home the life-threatening illness that has been killing people around the country," explained Fruit, who added, "the only thing worse than that, is knowing I gave it to my kids."
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Too young to be vaccinated, their seven and nine-year-old kids had congestion and low-grade fevers. But Fruit, who has a history of sinus infections, was sick in bed.
"I had never felt pain that bad in my head in my life. With my medical history, I'm pretty confident that if I hadn't had the boost from the vaccination, I might not be standing here talking to you right now," he said.
The Centers for Disease Control and Prevention does say there is some evidence that vaccination may make illness less severe in people who get vaccinated but still get sick.
"Despite this, some fully vaccinated people will still be hospitalized and die. However, fully vaccinated people are much less likely to be hospitalized or die than people with similar risk factors who are not vaccinated," the CDC said.
Fruit and his family are OK now, but they're part of a shifting pattern in breakthrough COVID cases.
The CDC has said "vaccine breakthrough cases are expected."
"COVID-19 vaccines are effective and are a critical tool to bring the pandemic under control. However, no vaccines are 100% effective at preventing illness in vaccinated people. There will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from COVID-19," the CDC said.
As for the variants, the CDC said, "New variants of the virus that causes COVID-19 illness are spreading in the United States. Current data suggest that COVID-19 vaccines authorized for use in the United States offer protection against most variants. However, some variants might cause illness in some people after they are fully vaccinated."
Our sister station KGO-TV reports, of nine COVID patients at Zuckerberg San Francisco General Hospital this past week, two were vaccinated. Of 13 COVID patients in Marin County hospitals, two were also fully vaccinated.
"Right now, we're about one in five of our cases are among people who are vaccinated, before that was about one in ten," said Marin County Health Officer Dr. Matt Willis. "Obviously the concern there is that the vaccine may not be quite as effective against the delta variant."
Willis said the vaccinated patients in the Marin hospital are older with underlying conditions; he also said they're doing well compared to unvaccinated patients.
"This is not an indictment of the effectiveness of the vaccine, but I think it's a reminder to us that variants are real, they do represent a risk to us, and our best line of defense is still getting that vaccine," Willis said.
The CDC said, "Current data suggest that COVID-19 vaccines authorized for use in the United States offer protection against most SARS-CoV-2 variants currently circulating in the United States. However, variants will cause some vaccine breakthrough cases."
The CDC is continuing to monitor breakthrough cases.
As of May 1, the CDC transitioned from monitoring all reported vaccine breakthrough cases to focus on identifying and investigating only hospitalized or fatal cases due to any cause. This shift, they said, will help maximize the quality of the data collected on cases of greatest clinical and public health importance.
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As of July 6, more than 157 million people in the United States had been fully vaccinated against COVID-19, the CDC reported.
During the same time, the CDC received reports from 48 U.S. states and territories of 5,186 patients with COVID-19 vaccine breakthrough infection who were hospitalized or died. People aged 65 or older made up 75%.
"Vaccine breakthrough cases occur in only a small percentage of vaccinated people. To date, no unexpected patterns have been identified in the case demographics or vaccine characteristics among people with reported vaccine breakthrough infections," the CDC said.
Sean Fruit said he would get the vaccine again, but he'd do one thing differently when it comes to his mask.
"Even though I'm allowed to take it off, I shouldn't have taken it off," Fruit said.
ABC News contributed to this report.