Santa Clara and San Mateo counties are now seeing more COVID-19 cases than at any other time during the pandemic, health officials told their respective boards of supervisors on Tuesday.

The current seven-day average of new positive cases in Santa Clara County now exceeds 4,000 per day, Health Officer Dr. Sara Cody said. Those numbers don't include the COVID-19-positive antigen tests people are taking at home, which health officials have no way of tracking, she said.

Cody called the case rate a "dramatic and really breathtaking explosion of cases," adding that the peak of infections might not arrive for weeks. State models indicate it could be sometime in early February or later. After cases reach their highest level, hospitalizations are expected to peak.

"We're facing some very, very difficult weeks ahead," she said.

Wastewater surveillance, which the county uses to track the spread of the SARS-CoV-2 virus causing COVID-19 infections, has increased sharply in all four sewersheds in the past two weeks, she noted. The current concentration has surpassed last winter's surge. On Friday, Jan. 14, however, Cody gave an update during a press conference that all four sewersheds are showing a downturn in numbers, a hopeful sign.

San Mateo County Health Chief Louise Rogers said the load of positive cases is a challenge.

"We are working through a difficult period in every sector, resulting in strains on the systems we all rely on. This is the highest number of cases we have ever experienced," she said.

San Mateo County's rate soared from an average of 79 new cases per day last week to 905 this week. On some days, the county had between 1,000 and more than 1,500 positive cases, she said.

In both counties, people have been flocking to hospital emergency rooms for COVID-19 tests, when they failed to easily find test appointments elsewhere. Health care officials have been trying to redirect the test-seekers away from the ERs.

Cody and Rogers also said that vaccinations are significantly reducing hospitalizations. In a dramatic graph of hospitalizations by vaccination status of people ages 18 and older, Cody noted that the seven-day rolling average showed that hospitalizations among unvaccinated residents are 20 times higher compared with those who had been fully vaccinated in Santa Clara County.

Residents who didn't receive their boosters had twice the case rate of breakthrough infections over those who had received their booster shots, she said. So far, 789,129, or 60%, of eligible residents have received a booster shot. Overall, 82.5% of residents have been vaccinated.

Vaccines and boosters are "a very, very, very, very important part" of staving off infections and reducing the severity of illness in breakthrough cases. The data "shows how well vaccines reduce your chance of becoming a case," she said.

But Cody added the reputed "mildness" of the omicron variant can be deceiving. The word "mild" has a different meaning to doctors than to patients. When doctors talk of a mild infection, they are speaking about not ending up in the hospital or on oxygen, Cody noted.

"That's different than a layperson thinking they get to watch TV in bed and drink a hot beverage," she said.

So far, the death rate has been relatively flat, another possible benefit of vaccinations. Currently, Santa Clara County has five to 10 deaths per week. In early January 2021, when vaccines were not widely available, 160 residents died, Cody noted.

Cody and Rogers are cautiously optimistic the death rate in the current surge — the fifth since the pandemic began — will be significantly lower due to the high numbers of vaccinated residents in both counties and the omicron variant's possibly less severe infection. The next few weeks will show where the numbers are headed.

Rogers noted the hospital system could still be overwhelmed by the sheer numbers of people who become infected by the more transmissible omicron strain. A certain percentage of those people will have severe illness, she noted.

If there's good news, it's that transmission rates of COVID-19 are relatively low in schools. Most children are becoming infected at family gatherings, Nancy Magee, San Mateo County's superintendent of schools, told the supervisors.