Sonoma County discovers 39 fully vaccinated people were infected by COVID-19

Such instances are rare nationally and known as “breakthrough cases,“ meaning the virus has broken through despite the protection from inoculation.|

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At least 39 Sonoma County residents who were fully vaccinated against the coronavirus have subsequently contracted the infectious disease, with two of the cases known to involve a West Coast virus mutation.

The discovery local public health officials acknowledged Tuesday represents the first information released about county residents who have gotten completely inoculated, then two weeks or more later tested positive for the virus.

Such instances are rare nationally and known as “breakthrough cases,“ meaning the virus has broken through despite the protection from vaccinations.

Infectious disease experts say this category of infection can be expected, since existing COVID-19 vaccines don’t give people 100% immunity. The two most widely used vaccines, Pfizer and Moderna, are respectively 95% and 90% effective at preventing infection by the pandemic disease.

Dr. Michael Vollmer, a regional infectious disease specialist with health care provider Kaiser Permanente, said only a handful of breakthrough cases have been identified and connected to the more than 1.5 million COVID-19 inoculations Kaiser has done in Northern California.

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

County public health officials are closely tracking people involved in breakthrough cases and whether the infections are being caused by variants that are more virulent and transmissible, and potentially more resistant to vaccines. Public health staff will have as many of the specimens tied to the 39 cases as possible analyzed by genetic sequencing — a process known as genotyping — to determine the presence of potential variants.

“That’s why we’re sending them to be genotyped. Are we seeing one (virus) strain more than another in our breakthrough cases? We haven’t seen that so far, but we haven’t gotten back all our reports back,” said Dr. Kismet Baldwin, the county’s deputy health officer.

Health officials said Tuesday that contact tracing conducted on the 39 breakthrough cases has discovered no instance in which a fully inoculated person has transmitted the virus to someone else. Also, only one of the 39 cases involved someone who was hospitalized, and that person’s illness was not severe and the person was released after a short hospital stay.

“We’ve been monitoring the contacts of these people that have tested positive (after vaccinations) and there doesn’t seem to be any transmission from these folks, these breakthrough cases, to subsequent contacts,” said Jenny Mercado, a county epidemiologist. “That’s really the most important message, besides not having any severe outcomes.”

The county’s revelation about local breakthrough cases comes amid growing concern nationwide and globally about the spread of coronavirus variants, particularly the most dangerous South African and Brazilian strains, and whether the three vaccines now approved by U.S. regulators could be any less effective against the mutations.

Of the county’s 39 breakthrough cases, five specimens have been submitted so far to a state public health laboratory for genetic sequencing to glean more information. Of these, the further analysis has confirmed two of the cases to be the West Coast variant known as B.1.429, and the county awaits the analysis of the other three cases. The COVID-19 vaccines now on the market being administered nationwide have been found to be effective against such western variants.

Dr. John Swartzberg, an infectious disease expert at UC Berkeley, said a vaccine that is 90% or 95% effective will inevitably yield breakthrough infections.

“If you have 100 people vaccinated, you’ll have 5 or 10 are going to breakthrough,” he said.

The good news, he said, is that even with people infected after inoculations are likely to have some level of immunity to severe illness and are also less likely to transmit COVID-19 because they have less of it in their bodies.

The 39 breakthrough cases represent about 1% of the 3,600 positive virus tests in the county from Jan. 26 to April 10. A case is considered a breakthrough if a fully vaccinated individual tests positive for the virus 14 days after vaccine completion, Mercado, the county epidemiologist said.

The county only seeks genetic sequencing on virus specimen samples it can obtain, mainly through tests conducted at its public health lab. That limits the instances to detect breakthrough cases.

To identify potential COVID-19 variants, county officials conduct a random selection of four virus specimens a week and send them to the Centers for Disease Control and Prevention for analysis.

The county also sends certain other virus specimens to a state lab for genetic sequencing. They involve samples in which the county seeks to, among other things: identify variants in breakthrough cases; detect people infected a second time after testing positive over 90 days ago; and learn more about residents under age 55 who suffered severe virus conditions.

Thus far, only three variants of COVID-19 have been confirmed in Sonoma County, among eight infections overall tied to mutations. They include two cases of the so-called U.K. variant, or B.1.1.7, which is present in all 50 states and medical experts have predicted could become the dominant national strain; three cases of the West Coast variant, B.1.427; and three cases of the West Coast variant, B.1.429.

Local and regional public health and infectious disease experts say it’s crucial to continue aggressive vaccination efforts and pandemic prevention measures, such as wearing masks, even after inoculation to keep virus transmission low in the North Bay and across the state.

Dr. Lee Riley, a UC Berkeley infectious disease specialist, said variants like the U.K. variant are thought to be behind the fourth virus surge in places like Michigan and Florida. So far that spike of variant infections hasn’t occurred here in California, he said.

“California does have its own set of variants,“ Riley said. ”But they don’t seem to be contributing to any sort of a surge here.“

Swartzberg, of UC Berkeley, said he’s hoping the West Coast variants ultimately dominate the more troublesome South African variant, which has been identified in California. State public health officials said Tuesday that 14 cases of the South African variant have been identified in the state.

“I’m rooting for the West Coast variant to take over, because the immunity we get from our vaccine works very well against it,” he said. “The biggest concern I have is the (variant) we haven’t identified yet.”

You can reach Staff Writer Martin Espinoza at 707-521-5213 or On Twitter @pressreno.

For information about how to schedule a vaccine in Sonoma County, go here.

Track coronavirus cases in Sonoma County, across California, the United States and around the world here.

For more stories about the coronavirus, go here.

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