When the first COVID-19 vaccines began to arrive in Washington more than a year ago, the jubilation was evident. People traveled far at first to find a hard-to-come-by shot, with a hope that the end of the pandemic, or at least its deadliness, was near.

For those inoculated against the coronavirus, it was fairly uncommon to later test positive.

Then the variants hit.

These so-called breakthrough infections started popping up throughout the country last summer. As vaccination rates climbed and infections persisted, concern grew. At the time, the delta variant was overwhelming the region’s health care systems. Then the highly infectious omicron variant landed in the state and breakthrough cases nearly doubled.

By the end of January, Washington state had reported 342,195 breakthrough infections. About 75% of those had occurred within the prior two months.

“With 85% of eligible people in [King County] completing their primary vaccination series and with the more infectious omicron variant, we saw more people getting infected who were vaccinated as well,” King County’s health officer, Dr. Jeff Duchin, said during a news briefing last week.

“But this does not mean the vaccines are not working,” he added. “Although vaccination doesn’t always prevent against infection, it does provide measurable protection.”

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According to the state Department of Health, about 19% of people with breakthrough infections reported symptoms, while about 2% were hospitalized. Less than 0.5% died.

Unvaccinated COVID patients, meanwhile, are five to seven times more likely to be hospitalized because of the virus, depending on age.

The health department identifies a COVID case as a “breakthrough” if the person had received the last dose of the Pfizer or Moderna vaccine or the Johnson & Johnson vaccine at least 14 days before testing positive. The agency doesn’t currently include boosters or additional doses in its definition.

DOH noted that some people might have become sick soon after vaccination or right before vaccination, when the body hadn’t had time to fully build protection.

“The change in breakthrough rates is likely due to a combination of factors, including the fact that when omicron arrived, the vast majority of our population was fully vaccinated,” said Gabriel Spitzer, spokesperson for Public Health — Seattle & King County. “As more people are vaccinated, we expect a higher proportion of cases to occur among vaccinated people. For example, if everyone were vaccinated, all cases would be breakthroughs.”

The jump in cases could also be attributed to possible waning immunity, local changes in mitigation recommendations or different coronavirus variants circulating simultaneously, according to DOH’s most recent report on breakthrough infections, released last week.

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Visual reporting of local news and trends is partially underwritten by Microsoft Philanthropies. The Seattle Times maintains editorial control over this and all its coverage.

Public health officials have reminded residents that while breakthrough infections are possible with any vaccine, vaccinations and boosters still help prevent severe illness and hospitalization.

Detailed data comparing breakthrough infections during the delta wave to those during the omicron wave is limited, but a team of University of Washington researchers recently found that many of last year’s breakthrough cases induced very strong antibody responses, said Lexi Walls, a UW biochemical researcher who helped lead the study.

“But getting boosted is the best thing you can do to not only protect yourself against infection, but also protect against severe outcomes from delta and omicron,” said Walls, who has studied coronaviruses for more than five years. “Even though they’re wildly different from what you’ve been vaccinated with, which is the original SARS-CoV-2 strain.”

Breakthrough infections were also more common among younger people, state data shows, with the bulk of cases reported among those 20-49. While the age distribution has changed over time as more age groups became eligible for vaccines, breakthrough infections are significantly less common among people over 65.

Since last fall, when the state began collecting reinfection data, more than 32,950 Washingtonians reported a reinfection, with more than 90% recorded after omicron surged through Western Washington.

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State researchers have since found that omicron is responsible for four to eight times as many COVID reinfections, compared to delta reinfections.

Of those reinfected, about 2% were hospitalized and 0.2% died, according to DOH.

The health department attributed the rise in reinfections to an increase in the number of people with a “primary” infection, the spread of the omicron variant and waning immunity. 

Reinfections were more common among younger people, with most cases reported among 18-to-34-year-olds, the report said. People over 65 with reinfections were most likely to be hospitalized.

“Differences in testing due to workplace requirements, access to testing, and personal preferences regarding testing may be impacting reported reinfections by age,” DOH said in its report. Age-specific differences in vaccination behaviors and chronic diseases might have also had an impact, the report said.