The number of COVID-19 cases in California is steadily declining. Mask mandates are being peeled back. And Gov. Gavin Newsom recently unveiled "California SMARTER," a plan outlining a future of public health support during the next COVID-19 phase.

So what will "endemic" rather than "pandemic" COVID-19 look like? That depends on many variables, according to Stanford epidemiologist Dr. Yvonne Maldonado. Much of what will occur depends on what the virus itself will do.

At the moment, we're still in a pandemic phase of the health crisis, said Maldonado, a professor of pediatric infectious diseases and of health research and policy at Stanford.

"For now, we're in a period of declining cases and of increased immunity and there's no other variant of concern in sight" in California, she said. But "we could be heading into an endemic phase or be in the early stages."

The new subvariant of omicron — subvariant BA.2, which is 33% more transmissible than its predecessor — is present in the Bay Area, according to local health administrators, but Maldonado said thus far it hasn't spread widely and there isn't a huge spike in cases. If that metric holds and the number of cases, whether high or low, doesn't spike, then we'll likely be in an endemic phase of the disease.

Endemic doesn't mean the disease is disappearing from within a population. It simply means that the disease is widely circulating with a more or less constant level of cases that don't fluctuate wildly. Rather than fading away, COVID-19 will likely be something we'll continue to live with.

In general, diseases can be endemic to populations in some parts of the world but not others. Cholera is present and endemic in some countries but isn't prevalent in the U.S., for example. Tuberculosis is also an endemic disease and still infects one out of three people in the world, she said.

Influenza, on the other hand, creates a pandemic every year, since it has different variants that spread throughout the world. It is controlled, however, with updated flu vaccines that target the new strains annually. Influenza hasn't mutated to a benign disease since it still causes thousands of deaths each year.

COVID-19 did something that is relatively rare for a disease in the history of the world.

"It started in a completely susceptible world population" that didn't have any immunity, Maldonado said.

Whether a population enters an endemic phase depends on the virus and its variants, the sustainability of mitigation measures such as vaccines, and the attitudes of people toward the disease, she said.

An endemic SARS-CoV-2/COVID-19 virus could become highly transmissible but relatively benign, depending on how it evolves.

"The common cold can cause hundreds of millions of cases annually, but it doesn't put people into the hospital," she said.

COVID-19 could become influenza-like, with a high rate of transmissibility and the ability to kill many people. Up to 60,000 people die each year from influenza and its complications, she noted.

But people don't respond to influenza in the same way they currently do to COVID-19.

"We've learned to live with it or to ignore it," Maldonado said.

There's also a widely available and largely effective vaccine for influenza that is adjusted annually to help the body create antibodies against the most prevalent strains.

COVID-19 infection has thus far killed 1% of the world's population, an estimated 5 million people, though the true number of deaths is likely much higher, Maldonado said.

Vaccines against SARS-CoV-2 are highly effective against current strains, but they confer immunity only for a few months before their effectiveness begins to drop off. Immunity starts waning after four to six months for adults. In children ages 12 to 15, two doses offer protection for three to five months, after which a booster shot is needed, she said.

Variants could overcome the immunity afforded by the current vaccines.

Endemic COVID-19 will likely be managed with vaccine boosters along with masking, social distancing and all of the other known precautions that protect against the spread of the virus, she said.

It's not yet known how long boosters offer protection or if people will need another booster or sequential boosters. Perhaps a long-term booster would be possible against COVID-19, she said.

Tetanus requires a booster every 10 years, although it is a bacterium and isn't a virus, she said.

When the virus does reach the endemic phase, strategies such as the state's SMARTER plan will need to be nimble and flexible. The $3.2 billion emergency response package includes $1.9 billion of approved funding to equip the state's health care system with supplies and staffing.

Could an endemic virus flare up and cause a pandemic again? It's always possible, Maldonado said. It's also possible the virus could mutate or hybridize and create another pandemic.

"We could see SARS-CoV-2 co-infect with another virus and create another progeny virus," she said.

Virus mutations and transmissions from animals to humans, which some scientists think created the strain of coronavirus that started the pandemic, are also likely to continue, she said. Three recent scientific studies released on Feb. 25 and 26 indicate that SARS-CoV-2 originated in animals at the market in Wuhan, China. The studies, which are in the preprint stage and haven't yet been published in peer-reviewed journals, are adding to the long-held theory that the virus crossed over from animals to humans.

Maldonado said that such virus transmissions are more possible now because people are encroaching into land and animal habitats they didn't frequent before and come into contact with animals that have diseases that could mutate and infect people.

Maldonado said it's important to learn the lessons of this pandemic and how to deal with public health around the world. Increased funding, expanding public health programs and worldwide conferences to keep on top of emerging infectious diseases will be necessary to fight future pandemics.