Instead, the Bay Area has been snared in a sixth wave of infection since early April, with cases likely at or near their highest levels of the pandemic, though COVID hospitalizations and deaths have remained comparatively low. The new variants could prolong the current surge or cause a fresh spike in cases, and they could put pressure on health care systems frayed from nearly three years of pandemic stress.

Nationally, the spring, and now summer, surge has been fueled by a cascade of variants — all derived from the original omicron that emerged last November — that have cycled through the population at an astonishing pace, like sprinters passing a baton in a race. Each variant has been more infectious than its parent, and they increasingly are adopting mutations that help them evade immunity, too.

The BA.4 and BA.5 variants now taking over appear to be the most infectious and best able to evade immunity of any strain so far. Health experts worry that even regions that have endured large winter and spring surges could be susceptible to the new variants because so many people, vaccinated or not, will be at risk of reinfection.

“I think the likeliest outcome (of BA.4 and BA.5 taking over) is a stable plateau or moderate increase in cases over the next several months. And a lot of reinfections,” said Dr. Robert Wachter, chair of the department of medicine at UCSF. “People who got COVID in January/February, or in March/April, may believe that they have this ‘get out of jail free’ card, and it doesn’t actually work that well. They’re going to be vulnerable and probably not acting as though they’re vulnerable.”

BA.4 and BA.5 are genetically very similar — they carry almost identical mutations in the spike protein, the area of the virus that allows its entry to human cells — and tend to be lumped together in epidemiological reports. In the U.S., the variants combined accounted for just over half of all new cases as of the end of last week, according to the CDC. BA.5 makes up the larger proportion, accounting for about 37% of cases on its own.

The variants have mutated substantially from their omicron ancestor, to the point that some scientists believe that BA.5, at least, should be given its own Greek letter name. That genetic drift may partly explain why infection with the original omicron provides little protection against reinfection with its variants.

Even if reinfection becomes more common in the coming weeks, it’s unclear whether the Bay Area or other parts of the country will see significant increases in hospitalizations or deaths. Vaccine-induced immunity appears to hold up well at preventing severe illness, though that protection has waned somewhat with successive variants and may further erode with BA.4 and BA.5. Early laboratory and animal studies suggest the new variants could cause more severe symptoms, but many health experts say it’s too soon to know whether they’re more of a threat to humans.

In parts of the world where BA.4 or BA.5 have already taken over, the impact on hospitalizations and deaths has been mixed — in some places they increased only slightly, while in others they basically doubled over a period of several weeks.

“What will happen in the United States over the course of the coming weeks and months, based on what we’re seeing internationally, is in a lot of ways a crapshoot,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, in a podcast late last week. “We could find ourselves in a pretty challenging situation.”

The variants’ impact on health care systems may be the most important question to answer. But for many Bay Area residents who have been waiting several months for cases to drop again so they can feel a bit safer moving around in public — going to work, taking public transit, or maybe going to the movies or a restaurant — the short- and long-term outlook remains murky. Health experts say it’s almost impossible to predict whether the new variants will lead to another explosive rise in cases or simply drag out the current surge. They acknowledge that neither outcome is particularly appealing.

Dr. Benjamin Pinsky, head of the Stanford Clinical Virology Laboratory, noted that the community positive test rate measured by his lab — a key marker of the pandemic that helps define how much disease transmission is happening — has remained high but steady, around 11%, over several months, even as different omicron variants took hold. “It has encompassed the transition from omicron to BA.2 and all the different 2 lineages, and now onto 4 and 5,” Pinsky said. “This is different from what we’ve seen before, where cases went down to a manageable level for a month or two” before the next surge took off.

Pinsky and others noted that the variants alone won’t define how the surge unfolds this summer. Human behavior will also be a factor, especially now that public health officials have largely moved away from broad mandates. And many people who were recently infected may wrongly assume they are well protected now, and therefore be reluctant to take certain precautions like wearing masks or avoiding large indoor gatherings.

“At this point, that is almost the biggest question: Will people respond to a significant threat in the ways they have previously? You would guess no,” Wachter said. “Certainly from a policy standpoint, unless it gets dire and you see hospitals packed, you won’t see mandates come back.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday