Sonoma County public health officials revealed Tuesday that 39 people in the county have been so-called "breakthrough" COVID cases, meaning that they became infected with the coronavirus more than two weeks after getting their second vaccine shot.

But UCSF infectious disease expert Dr. Peter Chin-Hong cautions that this is an expected development and should not be taken as bad news. As he tells ABC 7, "That is exactly what we expected and in fact it's probably better than expected."

Out of more than 3,600 COVID-19 cases documented in Sonoma County during this same period of time, the 39 "breakthrough" infections represent around 1% of the total. And the vaccine trials of the Pfizer and Moderna vaccines found that they were both upwards of 90% effective at preventing infection, and even more effective at preventing serious illness.

"Nobody in the study when they got the vaccine got a serious disease, were hospitalized, or died, which is really important to know even if they weren't fully protected," Dr. Chin-Hong tells ABC 7.

Stories about these "breakthrough" infections will likely continue to make for sensational headlines, and will unfortunately fuel vaccine hesitancy among Fox News watchers and others.

But really, this should just be a reminder that mask-wearing is still essential until virus levels in the Bay Area are brought down closer to zero.

As the New York Times reported last month, initial studies that have followed fully vaccinated healthcare workers — the first cohort of Americans to be fully vaccinated as of January — have found very small numbers of "breakthrough" infections. In one study, four such infections were found among 8,121 fully vaccinated employees at the University of Texas Southwestern Medical Center in Dallas. In another, seven out of 14,990 workers at UC San Diego Health and the David Geffen School of Medicine at the University of California, Los Angeles became infected more than two weeks after getting both doses of a COVID vaccine. Those numbers bear out a seemingly consistent rate of 0.05% chance of infection after full vaccination in the first real-world studies of the vaccines, post-rollout.

In all 11 of the cases in those studies, infected people showed either mild symptoms, or no symptoms at all.

A CDC study that followed 3,950 vaccinated participants in six states over a 13-week period starting in mid-December found similar results. It showed that in real-world conditions with high virus prevalence, the two mRNA vaccines from Pfizer and Moderna were 90% effective at preventing infection two weeks after the second dose, and 80% effective two weeks after the first dose.

To date, despite the fact that these healthcare workers in the study were at work during a period of widespread virus transmission when virus mutations were prevalent as well, there has been no evidence that virus variants are any better at infecting the vaccinated than the original strains. And experts suggest that "breakthrough" infections could primarily be infecting people with immune systems weakened by medications or other conditions — meaning that the vaccines simply produced weaker immune protections in those patients.

Regarding the Sonoma County data, Dr. Michael Vollmer, a regional infectious disease specialist with Kaiser Permanente, tells the Press Democrat that the numbers are consistent with what Kaiser has seen across its entire Northern California network.

"This is real-world evidence that the vaccines are incredibly effective at preventing illness and are lifesaving," Vollmer says. "The more people we can vaccinate, the less opportunity the virus can spread current variants or create new ones."

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