San Francisco is nearing herd immunity, some experts believe, a milestone in California’s efforts to take control of the COVID-19 pandemic.

San Francisco has one of the highest vaccination rates in California, with 72% of residents receiving at least one dose. Only one other district in California – Marin, north of San Francisco – has a higher vaccination rate, with 75% of residents being at least partially vaccinated. In both San Francisco and Marin Counties, vaccination rates are well above the national vaccination rate of 56%.

Herd immunity, also known as community immunity, is a form of indirect protection from infection by people without immunity to disease. It occurs when a significant percentage of the general population is either immune to vaccines or has survived a previous infection.

There is no definite percentage at which herd immunity to COVID-19 is achieved. It can only be tested if essentially all restrictions are relaxed and officials watch to see if disease transmission increases.

Previous estimates of when herd immunity to COVID-19 will be achieved has been between 70 and 85% of a community with immunity, either through immunization or through disease survival. Despite the unclear parameters surrounding achieving herd immunity, it remains a primary goal for public health professionals to approach this by maximizing vaccination rates.

“I think San Francisco is pretty close,” said epidemiologist Dr. George Rutherford of UC San Francisco, who thinks a good guess that a herd immunity threshold is north of 75%. “I think we’re really well positioned here, we’re talking about the city [of San Francisco]. In other places we are very close. “

San Diego County also has one of the highest vaccination rates in the state, with nearly 70% of residents being vaccinated with at least one dose. In the Orange and Ventura counties, 57% of their residents are at least partially vaccinated; LA District, 55%; Sacramento County, 51%; Riverside County, 45%; and San Bernardino County, 41%.

Barbara Ferrer, LA County’s director of public health, said she doesn’t believe LA County has yet achieved herd immunity; She hopes it will get there by the end of summer.

“We have a lot of people who haven’t been vaccinated,” Ferrer said. Achieving herd immunity would mean daily coronavirus case rates are so low “that you have a lot of protection just because we vaccinated so many people”.

When herd immunity is achieved, suddenly nothing happens – daily coronavirus case rates are already extraordinarily low in San Francisco. And technically, herd immunity is achieved gradually: the more people become immune, the slower the disease transmission.

But once herd immunity is achieved and new cases of coronavirus are introduced among unvaccinated people, such as travelers, “it is very unlikely that they will spread for more than a generation or so. So you will not see these extended chains of transmission ”, assuming that unvaccinated people are randomly distributed in the population and not all come together.

Some experts have expressed confidence that after June 15, if the state fully re-opens its economy and lifts the lion’s share of business restrictions to contain the spread of the coronavirus, California will not slide back into a worse period of the pandemic in California counties kept at the least restrictive level, indoor restaurants and gyms at 50% of indoor capacity.

“We’ll see it on June 15th. If we stick to low cases – I really have no doubt about that, ”said Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco.

California already has exceptionally low COVID-19 hospitalization rates – there are about 1,000 people living with COVID-19 in the state’s hospitals, up from about 22,000 in January. The latest numbers mean a hospitalization rate of about 2.6 Californians hospitalized with COVID-19 per 100,000 residents, up from a peak of 56 per 100,000 residents.

The latest figure is significant, as Gandhi suggested in April that a good milestone for easing restrictions like masking and social distancing is for hospital admissions numbers to drop below 5 per 100,000 residents. During the height of the flu season, hospital admission rates for the flu averages around 20 to 40 per 100,000 residents.

Other countries with the world’s fastest vaccine doses per capita are showing promising patterns. Israel recently lifted rules that banned unvaccinated people from entering certain venues, Gandhi said.

On Tuesday, federal officials expressed concern about the surge in the Delta variant, also known as B.1.617.2, which was originally identified in India and has spread to 60 countries including the US and the UK.

Dr. Anthony Fauci, the U.S. government’s leading infectious disease expert, said the Delta variant was more transmissible than the Alpha variant, also known as B.1.1.7, which was originally identified in the UK and has since become predominant in the US . It may be linked to increased disease severity, such as hospitalization risk, compared to alpha, ”he said.

But luckily, studies show that two doses of the Pfizer and AstraZeneca vaccines provide sufficient immunity to protect against infection from the Delta variant, Fauci said. AstraZeneca’s COVID-19 vaccine is not approved in the United States, but is similar to the one made by Johnson & Johnson.