Want to achieve herd immunity? Get vaccinated.

A lot is happening on the vaccine front. To sort it out, UCHealth Today reached out to a vaccine specialist about the COVID vaccine and herd immunity.
April 21, 2021
man readying a covid vaccine in an effort to achieve herd immunity.
The United States is progressing toward coronavirus herd immunity, but that will not happen until more people are vaccinated. Photo Cyrus McCrimmon, for UCHealth.

The U.S. coronavirus vaccine rollout is pushing ahead at a pace of more than 3.3 million people a day. More than half the adult population and 80% of those 65 and over had received at least one dose as of April 16, according to the U.S. Centers for Disease Control and Prevention (CDC).

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Adding to the good news, CDC data show that just 5,814 “breakthrough” coronavirus cases had emerged among the 75 million Americans who had been fully vaccinated by April 13. Those numbers translate to a 99.993% chance that, once you’ve been fully vaccinated, you won’t come down with the coronavirus. Further, one-third of the 396 hospitalizations reported among these cases (396 cases being just 0.0005% of those vaccinated) were asymptomatic and admitted for other reasons. The same was the case for 12% of the 74 deaths – a death rate amounting to 0.0001% of those vaccinated. Overall, 30% of breakthrough cases were asymptomatic.

The Colorado Department of Public Health and Environment on April 20 reported 819 coronavirus cases among 1,489,481 fully-vaccinated Coloradans, meaning the vaccinated had 99.94% odds of avoiding COVID-19.

Despite all that – and the U.S.-approved vaccines’ proven safety – Monmouth poll results reported on April 14 found that about 20% of Americans aren’t planning to get vaccinated.

Ross Kedl, PhD, a professor of Immunology & Microbiology at the University of Colorado School of Medicine who answers questions about the COVID vaccine and herd immunity.
Ross Kedl, PhD, a professor of Immunology & Microbiology at the University of Colorado School of Medicine, answers questions about the COVID vaccine and herd immunity.

Given the infectiousness of viral variants, that 20% could be the difference between achieving the herd immunity needed to protect those to whom vaccines can’t be given (the very young, say) or whose bodies don’t respond to them (the immunocompromised, a group that numbers in the millions in the United States alone).

There is, in short, a lot happening on the vaccine front. To help sort it all out, UCHealth Today reached out to Ross Kedl, PhD, a professor of Immunology & Microbiology at the University of Colorado School of Medicine. Kedl, a vaccine specialist, has been a media fixture of late, including an appearance on a Rocky Mountain PBS special about coronavirus vaccines.

UCHealth Today: How good are these coronavirus vaccines?

Ross Kedl: These are some of the best vaccines that have ever been made, and that is not hyperbole. We generally consider a vaccine that’s anywhere upwards of 50-some percent effective to be successful. The yearly flu vaccine sometimes gets down below 20%, but if it bounces around 30-40% efficacious, then we’re reasonably happy with that, because even if it only blocks disease 30-40% of the time, we know that it prevents severe disease and that it also stops a lot of transmission.

So even in those percentages, we’re pretty thrilled, especially for something that’s very contagious. So when you’re getting up into the efficacy ranges these vaccines have for preventing severe disease and death, this has been knocked out of the park so far that it can’t even look back and see the park. These really are some of the most remarkable efficacy rates we have seen in vaccines since the beginning of vaccination with Edward Jenner in the late 1700s.

Q: So these vaccines stop people from getting sick. Do they also prevent COVID-19 transmission?

A: When they found these rates of efficacy of 70% to 90%, that was extremely reassuring. And what I think should have been said at that point was, “There’s never been vaccines this successful in the history of vaccinology that didn’t also block transmission robustly.” As I said, even with the flu vaccines that are 30-40 percent efficacious, we know that they have an impact on transmission as well. So it’s always been true that vaccines this good also shut down transmission very effectively. And instead, that wasn’t said. What was said was, “We have yet to prove it.”

A lot of that has to do with the fact that early messaging was so mismanaged when it came to prevention and distancing and masking. There was just extreme caution built into the “Well, we can’t prove it yet, so we’re not going to talk about what’s the very most likely thing to be true. We’re just going to say, ‘We don’t know.’”

The data are now pouring in [see this and this and this and this], and indeed, these vaccines are blocking transmission at rates of 90-percent-plus. Even if you did get infected after being vaccinated, there’s little to no chance you would transmit it. I think it’s worth emphasizing that one of the reasons you go and get a vaccine is that you’re going to assist in making sure the virus doesn’t go any further than you.

Q: You’re saying that, even if you ended up with a breakthrough case, you wouldn’t probably transmit it?

A: There are two things that happen in breakthrough cases. One, the disease you get is almost always extremely reduced relative to what it could have been. There are rare, rare cases – typically with extremely frail elderly – in which it’s worse than mild. But what is abundantly clear is that you have to be making a lot of virus inside of you before you’re capable of transmitting. So the second thing your immunity does, even if you get infected, is knock back the amount of virus you’re capable of producing. And almost always well below the amount that you have to make in order to transmit the disease.

There are a couple of studies that came out of the University of Colorado at Boulder [preprints here and here] that did a really good job emphasizing the fact that, if you limit the amount of virus a person produces even by a little bit, it can curtail transmission. So both of these are true of breakthrough cases.

Q: Is there a way to explain, in layperson’s terms, why these COVID-19 vaccines work?

A: Vaccines are essentially equivalent to going to the gym to work out. When you go work out, it doesn’t weaken your muscles. It builds them. Vaccines are a gym workout for immunity. In what way? Your immune system has always been built around the idea that it remembers the things it sees. Until vaccines came along, the only way that it could remember what it saw was by getting infected.

The question is, then, is there a way of exposing you to something in the infection ahead of time so that when your immune system actually sees it, it treats the infection as if it has seen it before. And that’s what a vaccine does. It essentially gives your immune system a full workout. And in this case, using just one component of the infection – the spike protein. And that component produces a kind of immunological muscle that lets you resist it should you encounter the infection later.

Q: Why do people often report feeling bad after a COVID-19 vaccination?

A: Much like going to the gym, you can get a little tired and winded. The first 24 hours after the vaccine is kind of the immunological equivalent of getting winded after hitting the gym. It gives you these sorts of flulike symptoms that are evidence of your immune system getting a good workout.

Those symptoms are not so much flulike symptoms as they are immune-like symptoms. When you feel lousy after an infection, it’s mostly because your immune response is kicked into high gear, and your immunity tends to overshoot – “more is better” in the grander scheme of things. It would be worse if it undershot, so it tends to overshoot.

When you get a fever, that’s actually almost exclusively because of a molecule that your immune system makes called interleukin-1. It’s not because of the virus itself. It’s because of the immune response to the infection. When you get super achy and you have a headache and your whole body sort of feels like it’s been hit by a truck, that’s actually from something called interferon. Again, it’s an immune molecule that’s overshooting in an attempt to protect you. That’s actually reassuring. It’s just a sign that your immune system is really “building some muscle”.

Q: What’s going on with reports of post-vaccination blood clotting?

A: It’s important to delineate between a couple of different types of clotting. There’s general clotting – a ton of people have clotting issues such as deep-vein thrombosis. These are actually disturbingly common medical conditions that many people experience on a day-to-day basis. That’s different than the extremely rare version that was seen with the Johnson & Johnson (J&J) vaccine.

As it applies to this rarer kind of blood clot, it’s one that’s seen more commonly among people who are given heparin. If you go to the hospital and they feel like they need to thin out your blood for whatever reason, they’ll give you heparin. About 3% of the time, though, giving heparin will generate a sort of bizarre blood clotting that’s actually dependent on having been given heparin. A rare frequency of people will clot because they make antibodies against a clotting factor bound to that heparin. What they found from people who were given J&J vaccine [and, in Europe, the AstraZeneca vaccine, which is similar to the J&J in many respects] was that one in a million people make these same sorts of antibodies even without being given heparin. So that was the surprise. Nobody saw that coming. And to this moment, we don’t quite understand why that’s the case.

You might imagine that, with people who make these antibodies when given heparin, the last thing you’d want to do is order heparin. With these vaccine-related cases, providers didn’t know that, so they gave them heparin. And that made it worse. The key thing is you don’t give them heparin, and now that they know that, I think they will find a successful therapeutic way forward to prevent these if/when they happen again.

(Editor’s note: On April 23, when the FDA ended an 11-day pause on administering the Johnson & Johnson coronavirus vaccine, FDA officials disclosed nine additional cases of blood clotting related to the J&J vaccine, bringing the U.S. total to 15 at that point. Three had died and seven remained hospitalized. All were women, 13 of them age 18 to 49. FDA officials called the risk of clotting “remote” but said J&J would add a warning to the vaccine noting the potential risk. That risk equates to 11.8 cases per million vaccinations among women ages 30-39 (one in about 85,000) and seven cases per million among women ages 18-49 (one in about 142,000).

Q: The coronavirus itself is also known to cause blood clots, isn’t it?

A: With the actual coronavirus, you have a roughly 20% chance of developing blood clots – a risk gargantuanly higher than any risk from the J&J vaccine. This vaccine-related clotting is connected specifically to two vaccine platforms [Astra Zeneca and Johnson & Johnson; Moderna and Pfizer vaccines have not seen such clotting]. Thankfully, this doesn’t happen for a good week after you’ve had the immunization, so there’s enough time to anticipate what’s going on. As I mentioned, now that they know what’s happening, I am confident they’ll be able to provide an appropriate therapeutic intervention. I really hope this serves to reassure people that they are watching closely. Even one-in-a-million side effects or potential side effects are being caught and managed.

Q: We’ve been talking about the COVID vaccine and herd immunity – the point at which enough of the population is vaccinated that those who can’t be vaccinated or for whom vaccines don’t work – for months. Do we know what that herd immunity threshold is?

A: There are a range of estimates, and I should note that I am not an epidemiologist and so admittedly not the most qualified to give the best answer here. That said, we know that we have remarkably good vaccines. But we also have an infectious disease that’s highly contagious, all the more so with these variants. It’s certainly going to be 70% or higher. I tend to think that perhaps 75% to 80% will likely do for maintaining a really low caseload in the community and broadly promoting herd immunity – assuming we can get kids into that vaccination schedule as well.

Q: You have noted how COVID-19 vaccination should have appeal across the political spectrum. Can you explain?

A: It’s to everybody’s advantage. If you’re an individual-liberties sort of person, well, the best way to protect your capacity for individual liberty is to get your immunity as high as possible, because that will get you the greatest range of freedom as time goes on.

If you are a public-health kind of person, and you’re about the common good, and the best thing for you is that the person next to you is as healthy as possible, then it’s also important to get this vaccine, because the best thing for the person next to you is that you have immunity. No matter how you break this down as far as political bent, it’s to your advantage to get yourself a vaccine. Also if you’ve got kids – and at the moment you can get your kid vaccinated down to 16 – it’s super important to get them vaccinated as well. Schools in general are just infectious-disease cesspools. They’re going to spread this around over and over again until we can get kids immunized and create some sort of barriers against transmission within that environment and in that age group.

About the author

Todd Neff has written hundreds of stories for University of Colorado Hospital and UCHealth. He covered science and the environment for the Daily Camera in Boulder, Colorado, and has taught narrative nonfiction at the University of Colorado, where he was a Ted Scripps Fellowship recipient in Environmental Journalism. He is author of “A Beard Cut Short,” a biography of a remarkable professor; “The Laser That’s Changing the World,” a history of lidar; and “From Jars to the Stars,” a history of Ball Aerospace.

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