While two companies, Moderna and Pfizer, have "stretch goals" to have a vaccine ready by November or December, Newsom tried to temper people's expectations of how available that vaccine will be.
"The bottom line is the first phases of the vaccine will be limited in terms of availability," Newsom said.
In the best case scenario, the governor said, those two companies will have a combined total of 45 million doses for the whole country by the end of the year. That may sound like a sizable number, but it quickly shrinks when you break it down.
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To start with, the vaccines will require two doses to be effective, the governor said, which instantly cuts the 45 million number in half. Then, when you consider California is just one of 50 states, Newsom expects the state would only likely receive between 1.5 million and 2 million of those doses.
"We have more people in our healthcare delivery system than that," said Newsom.
Healthcare workers and first responders will be among those prioritized to receive the vaccine, the governor said, as will other essential workers, those in skilled nursing facilities, incarcerated people, those over the age of 65 and racial minorities.
"Don't anticipate or expect you can go down to the local pharmacy and get your vaccination anytime in this calendar year," the governor said. "We don't expect mass availability until 2021. The question for all of us is: Is it going to be the first, second or third quarter?"
The answer to that depends on who you talk to, Newsom said, and how optimistic they are.
"I understand the prevailing winds, why people are pushing optimism. Because we're desperate for it," Newsom said. "When can I get my kids back to school? When can I go back to work? When can I reopen my doors of my business?
"And the sober reality is the expectation should not, cannot be this calendar year for the overwhelming majority of us. ... That will not happen until 2021."
He added the state is doing early planning to make sure it's ready to distribute a vaccine and store doses properly once it is ready.
The governor also announced a task force of 11 experts that will independently review all FDA-approved vaccines.
California has more than 867,000 known COVID-19 cases to date, but the rate of new cases has gone down over the past several weeks. The positivity rate, or the percentage of those who get tested for coronavirus and turn back a positive result, has been around 2.5%.
According to the California Dept. of Public Health, the "R effective number" or rate of spread shows in the last 10 days COVID cases have declined slightly from .99 to .96. In other words, spread is considered stable.
"We're doing the right things, we've decreased the reproductive number. It's all going in the right direction," said UCSF lead epidemiologist Dr. George Rutherford.
WATCH: UCSF doctor warns people to 'hunker down' to avoid third wave in California
Dr. Rutherford said even though it's moving in the right direction, doesn't mean the threat isn't still there.
"We've been here before. We were doing great at the end of May and it got away from us. We don't want that to happen again," he said.
With the slowing spread of the virus, the state has allowed for more counties to move forward in the four reopening tiers. (More changes to the county map are expected to be announced Tuesday.)
When it comes to California's wildfire crisis, the worst was avoided over the weekend. It was feared that new fires would ignite amid hot, dry conditions, and winds would quickly fan the flames. While a grass fire burned on San Bruno Mountain in South San Francisco Friday, no new major fires started over the weekend.
FIRE TRACKER: Track wildfires across California with this interactive map
The August Complex Fire in Mendocino National Forest is 86% contained and the Creek Fire near Fresno is 61% contained.