Susan Bergmann spent her 61st birthday swimming in the Bay and eating pizza and cupcakes. Despite COVID-19, she wasn’t alone: she was surrounded by her husband, daughter and the half-dozen friends who have become her “pod” over the past few months. They swim and bike together, a strict group whose members have agreed not to come into social contact with anyone outside of the pod.

As the Bay Area begins to reopen, some counties are codifying the kind of system Bergmann’s group has been using since the beginning of the pandemic. In Alameda, Contra Costa, Marin and San Mateo Counties, public health officials are now allowing for “social bubbles,” which come with strict guidelines but a lack of enforcement.

In all four counties, social bubbles allow for up to 12 people to socialize outside together. Although it’s not required, the counties’ public health departments recommend that bubble members continue to wear face masks and practice social distancing while together.

The counties say that bubbles should be maintained for at least three weeks, and that each person can belong to only one bubble, although exceptions are made for children. Children can take part in an additional childcare bubble, and children living in two different households can be in bubbles for each.

“If we’re going to allow people to be in Costco, in a crowded setting … there’s been a lot of social need for people to connect with people outside their house,” said Erica Pan, interim health officer at the Alameda County Public Health Department.

The 12-person limit is based on practicalities: It aligns with the group size allowed for childcare providers since the beginning of the pandemic. The three-week time frame is based on COVID-19’s incubation period, which can typically stretch up to two weeks.

“That group becomes the universe for exposure if there is a case,” said Matt Willis, Marin County’s public health officer. “You’re trying to limit the contact investigation if there is a case in a social bubble or a camp.”

Health officials say that there’s essentially no way to police people who aren’t sticking to their social bubble. Instead, they’re hoping that the model will “add some guardrails to something that was already happening,” Willis said.

Similar systems have worked well in New Zealand since much earlier in the pandemic. And one study published in Nature Human Behavior supports the idea that the model may be effective in slowing down the virus’ spread.

Researchers led by Per Block, a sociologist at the University of Oxford and the Leverhulme Centre for Demographic Science in the UK, used computer simulations to test how the virus would spread through networks practicing different forms of social distancing.

“Usually, this is done with things such as rumors or gossip or information,” Block said. “We take this logic and apply it to the spread of diseases.”

The team compared three scenarios: interacting only with people who share common characteristics like geography or demographics; sticking to groups that already have strong social ties; and developing strict social bubbles.

Out of the three options, social bubbles performed the best.

Social bubbles “create barriers to the spread of disease that hopefully can’t be crossed,” Block said: “If one person within that bubble is infected, it can spread to the other people in the bubble but not further. If no one is infected, it should be very difficult for any of us to contract the disease because we do not have any sort of face to face contact outside of this bubble.”

The models accounted for the fact that people won’t be perfect about sticking to their social bubble. That allowance is important, according to Stanford immunologist Robert Siegel. “The pod is only as safe as the least safe member,” he said. “It has less to do with the science of it and more to do with the behavior of the individuals involved.”

Dublin resident Stacey Fultz and her family have been in a social bubble with a neighboring family since the beginning of the pandemic.

Before forming a bubble, the families discussed their beliefs and their risk factors. The adults work from home, although their three kids have service jobs outside of the house. None of the families have any immunocompromised members.

The families often barbecue together, without masks but with plenty of handwashing.

“Social distancing is probably two to three feet,” Fultz said. “No real hugging — there’s elbow bumping. No sharing of utensils or water glasses or anything like that.”

Bergmann’s bubble — six women, ranging in age from their 20s to their 60s — has a 6 p.m. meetup spot in the Bay every day for whoever can come and swim.

For Bergmann’s birthday, a couple husbands came along too.

“It was a really fun birthday even by regular standards,” Bergmann said. “And especially by quarantine standards.”