Recent data reveal origins of local spread
Officials: Virus caught from car pools, work transmitted at home
Silicon Valley has contained coronavirus outbreaks better than most of urban California and the U. S., but it still reports an average of more than 100 cases a day.
So how are they getting sick? Recent data from Santa Clara County health officials suggests people here are catching the virus at work or in car pools and spreading it to others at home.
That may be a bit surprising, Santa Clara County Health Officer Dr. Sara Cody said in an update this past week to county officials, given that scientific literature and reports across the country have highlighted large gatherings — a religious service with singing, a motorcycle rally, a political event — where a single infected “super spreader” passes the virus to many others who aren’t family or close friends.
Not here in the Bay Area’s largest county, where such mass events mostly aren’t allowed. The new data also provided a window into jobs where infections might be more prone to spreading.
A review of 24 work-site clusters involving three or more COVID-19 cases within a two-week period from Aug. 6 to Sept. 30 showed:
• One in four of the clusters examined involved a construc
tion site.
• Food service and restaurants accounted for 21%, as did retail businesses.
• Just one outbreak each, or 4% of the total, was reported among workers at a place of worship and at groceries — one of the most highly trafficked types of workplaces.
Cody said the survey shows people who have spent the pandemic trudging to and from work and sticking close to home may have a false sense of security.
“In talking to people with infections we discovered that people have this perception that the risk from people that they know well and feel comfortable with like family members, relatives, friends, is much lower than that from strangers,” Cody said. “It’s human to think that … there’s a perception that the threat is less, when actually it’s maybe greater.”
Supervisor Cindy Chavez, president of the board, said the findings raise concerns about measures in place to reduce transmission among workers at job sites — she said she’s noticed construction workers without masks — and the ability of people in large households to isolate after one of them gets sick.
“When we have asked people if they are able to quarantine at home, that presumes that their home is roomy enough for them to do that,” Chavez said. “That’s something that we’re going to have to address.”
Santa Clara Count y, home to technology titans like Apple, Intel and Adobe, saw some of the first U. S. COVID-19 cases dating back to January and led the Bay Area in mid-march with the country’s first lockdowns on schools, businesses and other public activity to slow its spread.
Its outbreaks were soon after overtaken by those in other Bay Area and Southern California counties, and today it is among a few large urban counties with outbreaks in the state’s “orange” or “moderate” tier. But despite maintaining restrictions that go beyond what the state requires, the county of nearly 2 million residents still sees a steady flow of new cases.
Most of those occur not among the county’s highpaid tech elite who have the luxury of working from home over computers but in the eastern and southern parts of the county. Those lower- cost areas are home to much of Silicon Valley’s “essential” service and retail workforce — construction, food production, groceries, auto services, transportation, delivery, security, as well as retail, restaurants, salons, landscaping and the like.
They’re also home to most of the one in four county residents who are Latino, who account for well over half its COVID-19 cases — the only demographic in the county with a significantly higher share of infections than its share of the population.
Cody said county health officials recently examined 61 recent cases from its eastern and southern regions, ages 12 and up and 80% Latino, to learn how the contagion spread to the infected.
Unlike the usual contact tracing, which tries to find a case’s recent close contacts to warn that they were exposed and should be tested to break the chain of transmission, the interviews looked backward to learn how the case became infected. And they further asked if they knew how the person who might have infected them got sick.
Of the cases officials interviewed, 34% said they were infected by someone in their household, 31% by extended family and friends at informal social gatherings, and 22% from the place where they work. The majority of cases, 56%, also reported that the person who infected them was infected at work, while 23% said that “seed” case got infected at an informal social gathering among family and friends.
While the biggest outbreaks nationally have been tied to super-spreader events in which one highly infectious person who typically isn’t feeling sick at the time attends a gathering and infects several others, the virus also spreads one or two at a time through prolonged close contact among households and coworkers.
Cody said that the “attack rate” or likelihood of an index case passing the virus to others is anywhere from 10% to 30% in households and 8% to 11% at work.
“Once this virus is introduced into households
it spreads very easily to other household members,” Cody said, adding that family and friends tend to let their guard down on masks, social distancing and the like when they get together.
But despite the recent data, the county still reports that since May 22, just 42% of cases are linked to contact with a known infected person, and 5% to outbreaks, while for 53% of the county’s cases, it is unknown how they became infected.