COVID-19 'breakthrough cases' are 'extremely unlikely,' experts say

More than 100 people in Washington state have contracted COVID-19 after being fully vaccinated against the virus, according to officials from the Washington State Department of Health.

Known as "breakthrough cases," these illnesses are expected with any type of vaccine, officials said. The department reported in a press release on Tuesday that, out of 1 million fully vaccinated people in Washington, there is evidence of 102 breakthrough cases in 18 counties since Feb. 1 — or 0.01 percent of vaccinated people in the state.

A worker fills a syringe with the Pfizer COVID-19 vaccine at the Yakima Valley Farm Workers Clinic in Toppenish, Wash., Thursday, March 25, 2021. In Washington state, workers who labor in crowded conditions such as meatpacking facilities and farms and orchards became eligible for the vaccine earlier in the month. (AP Photo/Ted S. Warren)
More than 100 people in Washington state have tested positive for COVID-19 after being fully vaccinated against the virus. Doctors explain that "breakthrough cases" can happen. (AP Photo/Ted S. Warren)

The majority of the patients with confirmed breakthrough cases had only mild symptoms, if any. However, the department noted, eight people with such cases have been hospitalized. The department is also investigating two potential breakthrough cases in which patients died. Both patients were over 80 years old and had underlying health issues.

"It is important to remember that every vaccine on the market right now prevents severe disease and death in most cases," the state's secretary of health, Dr. Umair A. Shah, said in the press release. "People should still get vaccinated as soon as they are eligible, and encourage friends, loved ones and co-workers to do the same."

"It's misleading to just talk about the number of breakthrough infections rather than the percentage — which is 0.01 percent," infectious disease expert Dr. Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Life. "No vaccine is 100 percent protective, and it is extremely unlikely, as evidenced by the percentage, that a breakthrough infection occurs."

According to clinical trial data, the Pfizer-BioNTech vaccine is 95 percent effective at preventing symptomatic COVID-19 infections, the Moderna vaccine is 94.1 percent effective and the Johnson & Johnson vaccine is 66 percent effective (but 100 percent effective against hospitalizations and deaths).

But people also have individual responses to a vaccine, pulmonary critical care expert Dr. Reynold Panettieri, director of the Institute for Translational Medicine and Science at Rutgers University, tells Yahoo Life. "Not all individuals get the same immune response to a vaccine, especially the elderly and those with immunosuppression," he says.

Dr. William Schaffner, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, agrees. "People who are frail and immunocompromised won’t get the benefits of 95 percent protection because their immune systems may not respond as well," he says. "Instead, they may get 80 percent protection."

There’s also the possibility that a COVID-19 variant could "evade protection of the vaccine, in part," Schaffner says. (Health officials in Washington state said they are trying to identify patterns in the people who had breakthrough cases there, including whether a particular variant of the virus may have caused the infections.)

Doctors stress that while breakthrough infections can happen, they're not common and usually not serious. "Most breakthrough infections, when they do occur, are mild," Adalja says.

Over time and as more people are vaccinated against COVID-19, it's expected that these breakthrough cases will become even less common, Schaffner says. "The virus will have more difficulty finding new people to infect as more people become protected from the vaccine," he says. "That’s the whole concept of herd immunity."

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