Epidemiologist on when SoCal might reach herd immunity, and why teens should be the focus of vaccinations

Written by Amy Ta, produced by Brian Hardzinski

A person is vaccinated by the U.S. military at the opening of the country's first federal and state operated community vaccination site during the outbreak of the coronavirus disease (COVID-19) in Los Angeles, California, U.S., February 16, 2021. Photo by REUTERS/Mike Blake/File Photo

In Los Angeles County, large-scale vaccination sites like Dodger Stadium are expected to reopen on Tuesday — after COVID-19 vaccine shipments were delayed in getting here due to storms. Meanwhile, coronavirus cases, hospitalizations and deaths have all been declining in LA County. Public health experts are starting to have realistic conversations about reaching herd immunity.

KCRW talks about this with Dr. George Rutherford, professor of epidemiology at UC San Francisco. 

KCRW: Why are cases dropping so quickly in Southern California, which was the nation’s COVID infection epicenter not so long ago?

Dr. George Rutherford: “It's because there was so much infection, and there's a lot of naturally acquired immunity. That coupled with the kind of basic public health things we talked about all the time, like masks and social distancing. And the rollout of vaccines has sort of tipped the balance. And I think we're going to continue to see this as long as things don't go wrong.”

There’s a statistic floating around that 50% of LA County residents were infected. Is that accurate?

“No, I think it's true for parts of Los Angeles. If you look at areas like Pacoima, and Boyle Heights, and East Los Angeles where 20% of the adults have been reported with COVID, you have to figure out how many more people have been infected than there is for each case that gets reported. I tend to use a number somewhere between two and two and a half. CDC uses a number closer to four, which I think is excessive in this case. 

… Pacoima for instance, I would not be at all surprised if we were to … survey everybody and find out that 50% of people have been infected. That's enough to really start tipping the balance here between ongoing transmission and less transmission.”

It might be safer to be in a place where there were high infections?

“It depends on how much infection is ongoing. So you're going to have greater population level immunity in those places. But if there's a lot of transmission still going on, and 50% uninfected is still a pretty big chunk, it might be less safe there than, say, Catalina. 

Governor Newsom said in Long Beach today that there are not enough vaccines to accommodate the need and demand. What about that? 

“The governor is absolutely right. … Part of it is the weather. Okay, not much we can do about that — short of fixing climate change. And part of it’s demand, something that we're trying to fix, and that the state and the federal government have taken on, quite seriously trying to reassure people who are hesitant about vaccines. 

And then part of it is getting the supply out. I think the logistics are pretty much in place. But you have to have vaccine to give at the end of the day in order for it to be successful. 

And then we have production delays and shipment delays and stuff like that. It slows down the distribution and slows down the time until we can get to true herd immunity.”

What will cause this disease to finally start receding? More herd immunity than vaccination? A combination? 

“Herd immunity includes vaccination. So it's both naturally acquired immunity plus vaccine immunity. … If we're really seeing declining transmission, and we don't get overwhelmed with variants that can break through … immunity, which we have no evidence yet in the United States, as long as all those things are [held] into place, it's trying to get as many people vaccinated as possible. We're not going to get much of a boost from naturally acquired immunity, which in fact may wane, which is why people who've actually had the disease should get vaccinated when their turn comes.” 

A Johns Hopkins physician says he expects COVID to be mostly gone by April because of the widespread natural immunity from all the infections. What do you make of that?

“I think that they've overestimated the ratio between reported cases and numbers of infections. As I said, CDC uses what I think is a very high number, the bottom range of which starts at four and goes up. In our studies in the Bay Area … for every one reported case, there's one symptomatic case or asymptomatic case, or maybe one and a half asymptomatic cases. … And I think that's probably a lot closer to what's really truly going on. 

Early on in the epidemic, absolutely there were lots more infected cases than there were diagnosed cases. But now I think it's a lot closer to one to one.” 

Is that because there wasn't as much testing early on? 

“A million reasons. It wasn't recognized. … Cases weren't getting reported because people didn't know what was going on. … It's been stable for several months. And I think really, now we're looking at ratios closer to one to one.” 

Children younger than 16 aren't expected to be vaccinated for quite some time. Maybe not until the end of this year or maybe into next year. So how is that going to complicate things?

“I think you need to concentrate on teenagers mostly. They're the ones who are most likely to transmit. … The Pfizer vaccines are currently authorized for use in 16 and 17 year olds. 

… There are trials that are ongoing now for children 12 and older. I would expect those results … in the mid to late spring. And at that point in time ... we could expand vaccination programs to those groups as well. 

… I think there's a place for prioritizing high school-aged kids as … high schools start to open up again. 

The 12 to 18 year olds with the Moderna vaccine, I would expect those would be ready and authorized by the summer. 

The trials for younger children, which right now are enrolling children from 5 to 11, probably won't be ready, and we won't have results for those until the fall and get authorization then. And we could start vaccinating children in the fall. 

… We've now studied this for a year, and it's just not the case that younger children are big amplifiers of infection.”

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