COVID-19 vaccines / Photo by Matt Thomas for the San Diego Padres

Shane Crotty and his team at the La Jolla Institute of Immunology have been studying viruses for a long time. We at VOSD wanted to get his input again, as a local expert and national leader on vaccines.

He first joined our podcast in March when vaccines were first becoming available and we knew a lot less. Now, after a few months and a few major variants of the virus, we have new questions. Like, why is Delta so much worse? Is it OK to go to concerts and ball games? What’s the deal with booster shots? 

Crotty and VOSD Podcast host Scott Lewis get into all of that and break down some high-level science on COVID-19 that you should know now, as the county is still recovering from the surge of Delta cases in August.

Here are potions of that interview, edited for clarity and length.

Scott Lewis: There are a lot of questions right now about breakthrough cases. Personally, I have a couple of friends — two friends, they’re real people — and they’re both fully vaccinated and they both got COVID-19. They didn’t go to the hospital, but they both got pretty sick and it made a lot of people ask a lot of questions. It worked out and I think that’s kind of the point. They got it and they had the vaccine and that helped them get over it. But would you call what they experienced a breakthrough case?

Shane Crotty: Yes, definitely. I think what’s been most confusing to people is that it’s counted as a breakthrough case even if it’s just a PCR (Polymerase Chain Reaction) positive and there are no symptoms. There have been baseball players, for example, who have had breakthrough cases and all it meant was that they had a positive and they didn’t necessarily have any symptoms. And then there are people who have breakthrough infections that are symptomatic. The majority of these cases are in the asymptomatic and paucisymptomatic, or cold-like range, with sort of flu-like symptoms — a knock you out for three-to-seven-days kind of illness. Those are definitely happening.

SL: When we started to get the vaccines out and have these kinds of conversations about how they would work, did you think that these cases would happen as much as they are happening?

SC: I think the initial vaccine clinical trial results were so amazing, like 95 percent efficacy against measurable symptoms, both from Moderna and Pfizer. That was really exceptional. And then the real-world data, through the spring with the Alpha variant in the U.S. and elsewhere, that vaccinated people did not get sick, did not get infected. It was as close to a perfect vaccine as you could hope for. And then I think between a combination of the Delta variant being a lot more transmissible, and being somewhat more virulent and also just more time passing, we’re now in a different situation. A lot of it is Delta, that it is a tougher virus. So there are more vaccinated people having breakthrough infections. 

I think the public health messaging has been that almost all of those breakthrough infections are safe in the sense that it’s highly unlikely those people are going to have to go to the hospital. Even if they do go to a hospital, they will be coming back from the hospital, from a severe illness standpoint. It’s absolutely a pandemic of the unvaccinated, where virtually all of the deaths in the U.S. are in unvaccinated people, and a majority of the cases by a long shot are in unvaccinated people.

SL: So there was a study about the vacation town in Massachusetts, where a lot of vaccinated people spread the virus. Now to your point, very few of them went to the hospital and had trouble with it. But there was this kind of shock wave that there were vaccinated people who, because it was in their throat and nose, were able to spread it to other people, sometimes vaccinated people. And I think it kind of pierced this myth that the vaccine was some sort of force field, and that it’s not what we thought it would be because it could get into your nose and spread to other people. What kind of perspective could you put on what is actually happening in that sort of situation? And how likely is a transmission from vaccinated to vaccinated, or especially from vaccinated people to, say, unvaccinated kids?

SC: Where we were in April, May and the between times, the data was just that these vaccines were amazing at essentially stopping any infection and that there was really no evidence of vaccinated people pretty much ever transmitting. So it was really simple for places like the Centers for Disease Control and Prevent to have a recommendation of saying you don’t need the mask. You know, you don’t need anything. The vaccine is there doing it all for you. But Delta came along and Delta is dramatically more transmissible. 

It’s almost like thinking about it as a different virus. And one of the things that started being noticed in a couple of places was that vaccinated people who had breakthrough infections with Delta had similar amounts of virus or similar viral loads to unvaccinated people. So that’s a little tricky because that’s basically these PCR tests and it’s measuring viral RNA, which is a surrogate for how much virus is around, but not perfect. So those things, plus the outbreaks, triggered this statement by the CDC that vaccinated people could transmit just as much as unvaccinated. And that was inaccurate. 

What they were trying to communicate was that there was a change. That last month, it really wasn’t happening at all. And then this month, at the time with Delta, vaccinated people certainly could transmit. But when people have done kinetic studies, sampling over a period of days, the vaccinated people have the virus for a lot shorter periods of time. And so that’s also going to just narrow the amount of transmissions. Because instead of somebody having a virus they could be shedding for a couple of weeks, it’s probably for a couple of days. So it’s at least half as long, just to be conservative.

SL: It’s kind of like a tautology then. If you’re vaccinated and infected, you’re infected in the same potential way as somebody who’s unvaccinated and infected. You’re going to be able to spread it. But you’re probably far less likely to be infected.

SC: You’re far less likely to be hospitalized because your immune system is actually responding and cutting off that infection, which is also going to cut off transmissions. But then the additional piece that’s come up in the past few weeks is that some labs have tried to isolate infectious virus from the nose of people who were vaccinated versus unvaccinated. And they have a harder time getting infectious virus from the nose of vaccinated people compared to unvaccinated. And that’s probably because the viral RNA levels don’t actually mean the same thing in a vaccinated and unvaccinated person, but also that you’ve probably got a virus that’s covered with antibodies.

So the vaccinated person who’s making these antibodies — it didn’t manage to totally stop the infection, but it is managing to shut down some of the virus. When you breathe, if there are already antibodies bound to the virus, that virus isn’t going to be infectious. You’re probably in the range of vaccinated people transmitting 91 to 97 percent less often. But if you do get symptomatic it’s probably just a two to four fold reduction in the likelihood of transmission. 

Basically, unvaccinated people are transmitting 10 to 20 times as often as a vaccinated person. And if you got infected, it’s 90 to 95 percent likely that it was an unvaccinated person who sent that virus your way. 

SL: Well, let’s translate that to the question that came more than any for you and for our conversation, which is that I’m vaccinated but my kids are not. They can’t get vaccinated yet. How do I handle all of these data points and how do I think of life right now? Because it feels really weird. 

SC: Yeah, it is really weird. You want to minimize those chances of transmissions. Certainly it looks to me like there’s a lot of data already that schools that are actually masking, and masking consistently, are doing quite well, and schools that aren’t are getting hammered. Even locally, I know of two high schools, and one’s been pretty serious about their policies and communication, and the other one hasn’t. Those two schools have had very different outcomes, which I think is enough to say that masking works really well. Even if you can’t vaccinate a bunch of the kids, at least in that environment, it works. 

SL: One of the things I said the other day on the radio — and I don’t know if I feel comfortable about this. I’m going on again tomorrow as well so I’ll address it after what you say — but we were having this conversation about people still getting sick and the fact that the Delta variant was just ripping through the population. I said something to the effect of, ‘I think it seems like everybody’s going to interact with coronavirus. The coronavirus is going to come into people’s nose’s everywhere. Everybody is going to have that experience. It’s just how well their body’s going to react to it.’ Is that fair?

SC: Yeah, absolutely. Usually my stuff on Twitter is a little more subtle. I think it was June that I tweeted, look at this point, you’re either vaccinated or you’re going to catch Delta variants. Whether you’re young or old, if you’re unvaccinated, it’s going to be the most likely thing you’ll ever have in your life in terms of an infection that puts you in the hospital. And I said that because the data from the UK was so clear at that point, that it was just so infectious, that you’re definitely going to be coming in contact with people who are going to end up having it. And then it’s a question of, are you protected against it or not?

At this point, you are either vaccinated or you are going to catch Delta.
And whether you are young adult or old, when you catch Delta unvaccinated, you will have the highest chance of being hospitalized compared to almost any disease you have caught in your life.

— Shane Crotty (@profshanecrotty) July 16, 2021

SL: Can I ask you just a series of yes/no questions about what you would do? I noticed you have kids, right? They’re vaccinated, though. 

SC: Yeah.

SL: Okay. So let’s pretend they’re a couple of years younger. They are not vaccinated. Are you comfortable going on an overseas trip? 

SC: I’m trying to think of a country. We’d be comfortable going overseas to Hawaii, because we did it.

SL: Concerts indoors?

SC: No. But we’re not really concert people.

SL: Ballpark or stadium outdoors?

SC: No. If we had LA Galaxy or LAFC tickets right now, I would probably still … well, if the kid really wanted to go and we wore masks, I’d be totally fine with it.

SL: Masks make a difference then.

SC: Yeah, masks definitely make a difference for me there. For most outdoor stuff, I’m fine with no masks. But Delta is definitely transmissible enough that it is transmitting some outdoors.

SL: Are you going to get a booster?

SC: That’s a good question. There are still some things I want to see before I would feel a need for a booster. But for people 65 and older, like my parents and my wife’s parents, I’m all in on boosters as soon as they’re available. I feel like they’re likely to be approved in the next month and I think they’re a wise decision for people in that age group. And I think it makes a ton of sense too for healthcare workers, as we were talking about before. So depending on your particular situation.

SL: What would hold you back? The question of whether you want to have another day of fever?

SC: Yeah, I’m a big fan of not getting sick. I’m a big fan of my productivity. I’m a pain to be around when I’m sick. I’m totally useless. But there are some hints of Moderna and Pfizer being different in this regard. And I just happened to get Moderna and the phase three clinical trial data there was that Moderna wasn’t really showing the same waning that Pfizer was. And so I’m kind of looking for a little more data as to whether the need is equivalent in both of those. If I had gotten Pfizer, yeah, I would get it when it became available.

But going back to your thing about the kids. If vaccines became available for kids, but it looked like doses were limited, no, I’m definitely going to hold off until there are plenty of doses for the kids. Because as I say, any dose you can get into an unvaccinated person is absolutely more valuable than getting a dose into a vaccinated person. So there’s no way I would want to slow down getting doses into unvaccinated Americans.

This and all other VOSD Podcasts are available on Apple, Google, Spotify or wherever you listen.

Nate John

Nate John is the digital manager at Voice of San Diego. He oversees Voice's website, newsletters, podcasts and product team. You can reach him at

Megan Wood

Megan is Voice of San Diego’s director of marketing. She is responsible for producing and overseeing strategies that extend the reach of the organization....

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