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San Francisco Moves Toward Collective Immunity Against COVID-19;  LA is not

San Francisco is moving closer to collective immunity, some experts say, an important step 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 having received at least one dose. Only one other county in California – Marin, just north of San Francisco – has a higher vaccination rate, with 75% of residents at least partially vaccinated. The rates in San Francisco and Marin County are significantly higher than the statewide vaccination rate of 56%.

Herd immunity, also known as community immunity, is a form of indirect protection of unimmunized people against disease from infection. It occurs when a large percentage of the general population is immunized either by vaccines or by having survived a previous infection.

There is no definitive percentage at which collective immunity against COVID-19 is achieved. It can only be tested when virtually all restrictions are relaxed and authorities observe if disease transmission increases.

Previous assumptions about when herd immunity to COVID-19 was achieved ranged from 70% to 85% of a community immune, either by vaccination or by surviving the disease. Despite the unclear parameters surrounding obtaining collective immunity, getting closer by maximizing vaccination rates remains a primary objective for public health experts.

“I think San Francisco is pretty close,” said UC San Francisco epidemiologist Dr George Rutherford, who believes a good estimate of the threshold for achieving herd immunity is north of 75%. “I think we are in very good shape here, that is to say here the city [of San Francisco]. Other places we are very close.

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

LA County Director of Public Health Barbara Ferrer said she did not yet believe LA County had achieved collective immunity; she hopes to get there by the end of the summer.

“We have a lot of people who are not yet vaccinated,” Ferrer said. Achieving collective immunity would mean daily rates of coronavirus cases would have fallen so low “that you have a lot of protection just because we have so many people vaccinated.”

Nothing happens suddenly when herd immunity is breached – daily rates of coronavirus cases are already extremely low in San Francisco. And technically, collective immunity is achieved gradually: the more people who acquire immunity, the slower the rate of transmission of the disease.

But once herd immunity is achieved, as new cases of the coronavirus are introduced among unvaccinated people, such as through travelers, “it is very unlikely that they will spread more than a generation or so. something like that. So you’re not going to see these extended chains of transmission, ”assuming that the unvaccinated people are randomly distributed throughout the population and do not all congregate together.

Some experts have expressed confidence that California will not fall back into a worse phase of the pandemic after June 15, when the state prepares to fully reopen its economy and lift the lion’s share of trade restrictions aimed at curbing the pandemic. spread of the coronavirus, which has kept indoor restaurants and gyms at 50% of indoor capacity in California counties at the least restrictive level.

“We’ll see June 15 if we stay at a low case count – I have no doubt we will,” said Dr. Monica Gandhi, infectious disease specialist at UC San Francisco.

California already has extremely low hospitalization rates for COVID-19 – there are about 1,000 people with COVID-19 in state hospitals, up from about 22,000 in January. The latest figures translate into a hospitalization rate of about 2.6 Californians hospitalized with COVID-19 per 100,000 population, up from a peak of 56 per 100,000 population.

The latest figure is significant, as Gandhi suggested in April that a good milestone for when restrictions like masking and social distancing can ease is when the number of hospitalizations drops below 5 per 100,000. At the height of the flu season, influenza hospitalization rates average 20 to 40 per 100,000 population.

Other countries with the fastest per capita vaccine deliveries in the world show promising trends. Israel recently ended rules prohibiting unvaccinated people from entering certain places, Gandhi said.

Federal officials on Tuesday expressed concern over the rise of the Delta variant, also known as B.1.617.2, which was originally identified in India and has spread to 60 countries , including the United States and the United Kingdom.

Dr Anthony Fauci, the US government’s top infectious disease expert, said the Delta variant is more transmissible than the Alpha variant, also known as B.1.1.7, which was originally identified in the United Kingdom and which has since become dominant in the United States. It may be associated with an increased severity of the disease, such as the risk of hospitalization, compared to Alpha, ”he said.

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

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