At the end of March in Los Angeles County, unvaccinated people were five times more likely to be hospitalized compared to fully-vaccinated residents. Across the country, evidence of the offshoot of the omicron variant is taking hold.
On his first day on the job, White House coronavirus response coordinator Dr. Ashish Jha reported more than half of the country is seeing a spike in COVID numbers, with the omicron subvariant BA.2 making up more than 70% of new COVID cases.
"The pandemic isn't over, and we're going to see cases of this virus spreading, and we have to continue to be vigilant and continue to be careful," said Jha.
He said vaccines and boosters are keeping many of those infected out of hospitals.
RELATED: Philadelphia becomes first major US city to reinstate indoor mask mandate
"So if you are boosted, there are still some breakthroughs, but people will do very well. That's the number one thing people need to be doing is making sure they're vaccinated and boosted," Jha said.
Despite a small uptick in the seven-day average of new COVID cases, L.A. County has not yet seen a rise in hospitalizations or deaths.
This may likely be due to the county's overall 75% vaccination rate combined with those who've had prior infections.
According to the county Department of Public Health, daily COVID infection numbers rose by an average of 3.1% per day over the past week. Over the past seven days, the county reported an average of 960 new cases per day, a 23% jump from the previous seven days.
The increase in cases has been blamed on rising spread of the infectious BA.2 subvariant of the virus. BA.2 is an offshoot of the omicron variant that fueled a winter surge in infections.
County health officials noted that the average daily rate of people testing positive for the virus remains relatively low, at 1% as of Monday, but the rate has ticked up slightly over the past week. Also still low are the number of COVID-19-positive patients in county hospitals -- 265 according to state figures, including 38 being treated in intensive care.
But officials noted that traditionally during the pandemic, increases in hospitalizations tend to come within weeks of a rise in infection numbers, followed by an increase in deaths. That pattern is already being seen in other parts of the country, including New York City.
While indoor masking remains optional in Southern California, Philadelphia will become the first large U.S. city to start re-implementing masking in indoor public spaces next week.
"I sincerely wish we didn't have to do this again. It's my hope that our actions today will slow the spread of COVID and help us avoid seeing our ERs, once again, get so crowded that people can't get timely care when they need it," said Cheryl Bettigole, Philadelphia health commissioner.
And this week, Columbia University joins a growing list of colleges moving to reinstate masking requirements due to an increase in positive cases. Other universities that re-started indoor masking include Georgetown, American and Johns Hopkins.
The federal transportation mask mandate for places like planes and trains is scheduled to expire April 18.
New federal COVID-19 vaccine data indicates the overall number of people getting vaccinated has more than doubled in the last week.
This is largely due to an increase in the number of Americans getting boosted. More than 56 million Americans over the age of 5 remain completely unvaccinated.
City News Service contributed to this report.