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Can La Jolla reach herd immunity against COVID-19? UCSD expert says it’s ‘not realistic’

Chateau La Jolla resident Butch Hansen receives his first dose of COVID-19 vaccine in January.
(Ashley Mackin-Solomon)

As more and more people are getting one of the available COVID-19 vaccines, a term that might be coming to mind is herd immunity. But can a community like La Jolla, which has a vaccination rate upward of 70 percent, achieve its own herd immunity?

According to the World Health Organization, herd immunity, also known as population immunity, is “the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.”

WHO states that it “supports achieving herd immunity through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths.”

How many people need to get vaccinated for that to happen has wavered. National experts put the number anywhere from 60 percent to 90 percent to achieve herd immunity.

“Ultimately the goal is herd immunity by vaccinating as many people as possible,” said Richard Garfein, a UC San Diego adjunct professor in the division of infectious disease and global public health. “I mean everybody — frontline workers, gig workers, everyone. If you don’t vaccinate everyone everywhere, you could still have a problem.”

As for the magic herd immunity number, it’s “unclear” and depends on the virus, he said.

“It comes down to how infectious a virus is and how effective a vaccine is,” Garfein said. “Seventy-two percent was an early estimate for COVID-19, but for things like the measles, it takes 90 percent coverage to get herd immunity. The COVID virus is less contagious than measles, but we still don’t know what the magic number is.”

The San Diego County Health & Human Services Agency posts a daily list by ZIP code of the number of residents who have received at least one dose of COVID-19 vaccine. Through May 1, 26,399 residents in La Jolla’s 92037 ZIP code had received at least one dose. Based on population estimates from the San Diego Association of Governments, that is about 73 percent of La Jolla’s eligible (over 16) population.

Whether it’s enough for La Jolla to reach herd immunity is not a simple question, Garfein said.

“If you were talking about a rural community with little inflow and outflow, you could look at some level of herd immunity and potentially accomplish it. The thing about La Jolla is we are such a mobile and diverse community … and there are going to be people coming in and out ... that are not vaccinated,” he said. “It’s hard to think about the ZIP code as being immune to infestation, because there is so much mobility.”

He added that “if there was high vaccination coverage, say 80 to 90 percent, you could assume if someone comes in that is infected that they are less likely to infect other people.”

The problem, Garfein said, is that variants are occurring, and people with a mutated version of the virus could transmit it to a vaccinated person. “Some viral mutations can be harmless; sometimes it makes the virus ineffective — we like those. But sometimes you have a mutation that makes the virus more effective in replicating itself; sometimes the mutation can change the appearance just enough that the immune system doesn’t recognize it as something it is immune against.”

Thus, he said, “trying to get to herd immunity at the community level is not realistic because of mobility and variants.”

On the flip side, he said, the risk of transmission is “20 times lower outside than inside,” so vaccinated La Jollans are in a position to socialize outside in 92037’s temperate environment.

On April 27, the Centers for Disease Control and Prevention issued new guidelines for those who are vaccinated, especially when it comes to outdoor activities.

Fully vaccinated people can:

• Visit with other fully vaccinated people indoors without wearing masks or physical distancing

• Visit indoors without masks or physical distancing with unvaccinated people (including children) from a single household who are at low risk of severe COVID-19 infection

• Participate in outdoor recreation and other activities without a mask, except in certain crowded settings

• Resume domestic travel and refrain from coronavirus testing before or after travel and from self-quarantine after travel

• Refrain from testing before leaving for international travel (unless required by the destination) and from self-quarantine after arriving back in the United States

• Refrain from testing and quarantine following a known exposure, if asymptomatic, with some exceptions for specific settings

• Refrain from routine screening testing if asymptomatic and feasible

The guidelines recommend against large indoor gatherings.

“What we know is, if all the people around you are vaccinated and no one is sick, you can be comfortable taking off the mask,” Garfein said. “If you are in a group setting with someone that hasn’t been vaccinated, you still want to take those precautions. Older people that have been vaccinated don’t want to take a chance because they might be in contact with someone carrying a variant strain.

“The more people who are vaccinated, the lower the risk of transmission. … No one is going to announce, ‘We’ve hit herd immunity, take your mask off.’ But when we decrease safety practices and don’t see a resurgence, we can resume normal activity.” ◆