Of course, little about this pandemic has stuck to the plan.
Between the emergence of new coronavirus variants, unreliable vaccine supplies and uneven access to the doses available, it may take months or even years longer than anyone would like to hit herd immunity. It's possible California, the nation and the world may never get there.
That would be unfortunate, but not necessarily disastrous. The vaccines at hand are powerfully good at preventing severe illness, even with the variants that appear to be somewhat resistant, and they can be retooled to match the virus as it mutates. Vaccines will almost certainly reduce hospitalizations and deaths from COVID-19, and that could make the pandemic much more tolerable in the coming months, long before it's technically over.
"I'm hopeful we will be vaccinating the general public in a broader perspective by early summer. And we should see correspondingly wonderful impacts by early fall," said Dr. Catherine Blish, a Stanford infectious disease expert.
Blish is still optimistic about herd immunity — even if she, like most of her peers, has no idea how long it will take to get there. "The important thing to remember is we're going to need to be patient," she said.
Herd immunity occurs when enough of a population is immune to a virus that it is no longer able to spread. Measles is a good example: Enough people in the United States are vaccinated that the virus causes a problem only when cases are brought in from other countries and happen to reach communities that are under-vaccinated, leading to local outbreaks.
It's not clear exactly how many people will need to be vaccinated to reach herd immunity with the coronavirus; Dr. Anthony Fauci, the nation's top infectious disease expert, has estimated 70% to 85%. But it's a moving target, based on how protective vaccines are against new variants and other factors, including the effectiveness of the vaccines on children and how long immunity lasts. Children are unlikely to be eligible for shots until early next year, but studies have found that they are not driving the pandemic.
Regardless of what the final percentage is, and assuming vaccine supply becomes more reliable, many parts of the U.S. could get close to herd immunity in six to eight months.
Along the way, counties should see a continued, notable decrease in hospitalizations and deaths. Some experts believe the vaccines are already helping, with fewer outbreaks in nursing homes in particular. The effects should become more apparent in the next month if counties continue to reopen the economy and hospitalizations don't climb again — though there's a risk of a fresh surge in cases if more-infectious variants get a foothold, public health officials warn.
Once communities reach herd immunity, new cases should fall fast. If the Bay Area were an island, after it had vaccinated about 80% to 90% of residents, then the pandemic could be over in two weeks, said Dr. George Rutherford, a UCSF infectious disease expert.
But global ties mean that even after the region or the state, or even the country, hits herd immunity levels, the coronavirus will remain a threat — but a less life-altering one. If communities can drive cases down to just a handful a day, then they can quickly isolate and contain new infections that are imported and prevent outbreaks.
"How would we live if there were 10 or 20 cases a day or even 100 cases a day in the United States? We would be enjoying life considerably," said Dr. John Swartzberg, a UC Berkeley infectious disease expert. The U.S. currently reports well over 50,000 cases a day. "That's what we can get to with herd immunity."
Assuming herd immunity is achievable, there are several variables that could slow it down. One unknown is how long immunity lasts after people are vaccinated. Some infectious disease experts fear immunity could start to wane in just a few months, which would mean the first round of people to be vaccinated will need boosters before the final groups get a single dose. But most experts believe immunity probably lasts longer than that, maybe years.
Variants could complicate things and could even make herd immunity an impossible goal if the virus mutates too fast for vaccines to keep up. But even in that case, the vaccines probably would prevent most serious illness and the coronavirus could end up similar to influenza, in terms of how it affects day-to-day living and the impact on health care systems.
The most concerning variants so far are from South Africa and Brazil, both of which have been identified in small numbers in the United States; two cases of the variant from South Africa have been found in the Bay Area. Those variants reduce the effectiveness of vaccines, and they also may be able to evade natural immunity from previous infection. That may mean more people need to be vaccinated or need boosters for the population to reach herd immunity.
"The current vaccines will still make a dent. They just maybe can't tout the 95% effectiveness necessarily," said Shannon Bennett, chief of science at the California Academy of Sciences. "But I would take 60% in a heartbeat."
The bigger concern with variants is what's to come — if the ones that already have emerged are able to partially evade vaccines, what happens if future variants escape completely? That reasoning is part of the rush to get as many people vaccinated as quickly as possible, because the less the virus can circulate, the less it can mutate into something more dangerous.
The vaccine rollout has been notoriously bumpy in the United States. Many county and city leaders have said they have the staff and other resources to be able to reach nearly all of those approved for vaccination by late summer. But those plans are based on supply, which is ramping up but has been hampered by everything from government disorganization to winter storms. It's also unclear what proportion of the population will decline to take the vaccine, even if it is eligible.
Many public health experts are ramping up calls to prioritize vaccine for communities hit hardest by the pandemic. Focusing on those neighborhoods is important for two reasons: to protect the people who have already suffered the most, and to speed up the climb toward herd immunity.
"You want your vaccinations to break cycles of transmission," said Dr. Kirsten Bibbins-Domingo, vice dean for Population Health and Health Equity at UCSF. "The virus has not been hiding. We know where it's been. So go there and vaccinate everyone there."
"The slow vaccine rollout, the rise of the variants, and then this extremely uneven distribution makes me really think we won't reach effective herd immunity in the short term, or maybe ever," she said.
Public health officials said they're aware of the need to vaccinate communities with the highest case rates, and many counties have opened clinics in those neighborhoods. Last week, health officials opened a one-day-a-week pop-up vaccination clinic in Gilroy, which has the highest rate of infections in Santa Clara County.
Gilroy resident Patricia Lugo, 57, showed up with her 83-year-old mother Wednesday so they both could get vaccinated; Lugo qualifies as her mother's primary caregiver. Lugo said she's kept them extremely isolated to avoid infection, but she was nervous about the vaccine being so new and about side effects. She'd made two previous appointments for the shot and canceled.
The arrival of the variant from South Africa in the Bay Area finally convinced her. Plus, she wants to set an example for her four adult sons, who have reservations about getting vaccinated. She knows that until they're all vaccinated she can't hug her children and grandchildren. And her community, too, can't recover.
"I want everybody to get it so we're all safe and our lives can get back to normal," Lugo said a few minutes after getting her first shot. "Today I decided to get over my fear. This is what I need to do."
Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday
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