For Christine Gerchow and millions of other Americans, “herd immunity” isn’t just an abstract scientific concept.

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“It could be life or death,” said Gerchow, 40, who receives treatment for chronic illness that may prevent vaccination from protecting her against COVID-19.

As long as some people remain hesitant to get the shot, the virus will continue to circulate – and could cause infection in others, such as Gerchow, whose immune systems aren’t vigorous enough to fully defend them.

They’re insulated only when so many other people are fully vaccinated that the virus can’t find enough people to infect, then stops its spread.

The vulnerable include elders, people born with faulty immune systems and people who must take immunosuppressant drugs for illnesses ranging from cancer to rheumatoid arthritis. Also at risk are people who would love to be vaccinated, but can’t because of life-threatening allergies.

“They need us to protect them, as we protect ourselves,” said Dr. Kelly Moore of the Immunization Action Coalition, which partners with the U.S. Centers for Disease Control and Prevention to distribute vaccine information.

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Experts say it’s unlikely that the COVID-19 virus will ever completely disappear. But the closer we get to “herd immunity” – estimated to be an 80% vaccination rate, at least – the safer they are.

The nation is making progress: More than half of adults in the United States have been inoculated with at least one dose of a vaccine. Bay Area residents are more receptive than many other regions of the country, with 87% of adults saying they’ll get fully vaccinated, according to a Public Policy Institute of California poll in March. Acceptance is even higher in Santa Clara County, where 90% of residents want a COVID-19 vaccine, based on new data.

Yet others remain hesitant. A Pew Research Center poll in February showed that about 30 percent of the U.S. population is either reluctant or will not get the shot. Skepticism about the vaccines— due to fear, mistrust or misconceptions – is driving hesitancy, according to the poll.

Inadvertently, they’re putting others in harm’s way.

A very small group of people can’t get vaccinated against COVID-19 for medical reasons. These include people with a history of severe allergic reaction to an ingredient in the vaccines and people who had a severe allergic reaction to a first dose, said Moore.

Elders, even when inoculated, might be at elevated risk, said geriatric specialist Dr. Mehrdad Ayati of Los Altos. Vaccines remain very effective for those age 65 or older, but our immune defenses seem to fade slightly.  Overall, the Pfizer vaccine is 95% effective but drops to 89% in those over the age of 80. Moderna’s vaccine is 94% in younger adults and 86.4% effective in older adults. The J&J vaccine was equally effective – about 66% – for all ages.

“To save other people, everyone should be vaccinated,” especially elder caregivers, said Ayati.

People who are immunocompromised – an estimated 3% of the U.S. adult population – also face potential danger. They can safely get the vaccine, although there’s no data on the COVID-19 vaccines’ effectiveness because they weren’t included in the initial clinical trials. But with other vaccines, immunocompromised people are known to be less likely to develop a robust antibody response than people with strong immune systems.

So if they get COVID-19, they are at greater risk of severe illness and death, doctors say.

People can be immunocompromised for varying reasons. They may be on medicines to suppress their immune system if they suffer from an autoimmune disease like rheumatoid arthritis or lupus. Or they may be getting chemotherapy for cancer treatment, which dials down the immune system. People who have received organ and bone marrow transplants are on anti-rejection medications, such as steroids, that reduce their natural defenses.

When vaccinated, their bodies produce antibodies – but not enough of them. How well the vaccines work will depend upon each patient’s overall state of immunosuppression.

“There’s a good chance I’ll still be at full risk,” said a vaccinated 52-year-old San Jose resident with multiple sclerosis, who asked that his name not be used for privacy reasons. Recent research suggests that, if infected, he’ll have only 20% of a normal antibody response – and may have no response at all.

Employed in high-tech, he is particularly concerned about younger colleagues who are healthy and don’t consider COVID-19 a grave threat.

“Given recent trends, that may mean I’ll never be safe,” he said. It may force him to reveal his condition to others, because “it seems likely I’ll be wearing a mask and minimizing social contact for the foreseeable future.”

Disabled by lupus and ulcerative colitis, another San Jose resident said “the immunocompromised cannot take a chance of even a ‘lesser’ version of COVID. … It is not that I, and others, are ‘cautious,’ we are trying to stay alive.”

Gerchow, who directs UC Berkeley’s Academic Talent Development Program, has relied on immunosuppressant drugs for more than a decade to control an autoimmune disease called vasculitis, caused by inflammation of the blood vessels.

“Conversations are needed,” she said. “I hope that we can be compassionate and curious about vaccine hesitancy, particularly from communities that have been marginalized or harmed by our public health system.”

Failing to vaccinate everyone, she said, “could be lethal for people who have chronic illnesses.”