Andy Larsen: Breakthrough cases of coronavirus are up in Utah — here’s what to know

We’ll likely see the number of Delta variant cases grow until a larger percentage of Utahns either get vaccinated or infected.

The COVID-19 vaccines are very good. Excellent, in fact. Honestly, they’re better than we thought they’d be.

Unfortunately, they’re not perfect.

Because the efficacy of the main coronavirus vaccines isn’t 100%, some people still get COVID-19 after being vaccinated. And, as the Delta variant takes hold as the major type of coronavirus in Utah, the number of “breakthrough” cases — in which people are infected after being vaccinated — is rising.

Sometimes we’re a little bit afraid to talk about breakthrough cases, because we don’t want to give the wrong idea about the vaccine. Yes, it works! But it’s important to also describe what happens when people get COVID-19 anyway after being vaccinated — how often it happens, why it happens, and how it might change your understanding of the risk you face in your everyday life.

I trust you, our intelligent Salt Lake Tribune readers, to be able to learn about breakthrough cases without jumping to inaccurate conclusions, so let’s dig in.

How often are breakthrough cases happening in Utah?

Breakthrough cases are up in a big way in Utah. Cases among vaccinated people are still much less common than those among unvaccinated people, but both are growing, due to the Delta variant and its increased level of contagiousness.

Basically, we’re seeing between 50 and 100 cases per day among the vaccinated, and 300 to 600 cases per day among the unvaccinated. Once you adjust for the size of the unvaccinated vs. vaccinated populations, we’re seeing five to six times more cases among the unvaccinated.

Here’s one way to look at it: Vaccinated people are getting infected at rates that are approximately equal to what we saw in Utah in late April and early May of 2020, while unvaccinated people are being infected at rates approximately equal to what we saw in September.

And that makes sense: that 5- or 6-to-1 ratio is what you’d expect to see when comparing a population immunized with vaccines that have an average of 90% to 95% efficacy, to an unvaccinated population that has seen about 30% to 60% already get COVID-19.

While our levels of vaccination and previous infection were enough to result in a significant downturn in cases early in the year, now that we’re dealing with the Delta variant, we’re in a less-promising situation.

Why are we seeing more cases?

If you’d rather avoid math, you can skip this explanation, but here’s why. Epidemiologists estimate the level of immunity needed to reach “herd immunity” with the formula 1-1/R0, where R0 is the number of people an average sick person infects.

With the standard coronavirus, we estimated R0 to be about 2.5, which means we needed about 60% of the population to be immune to reach herd immunity. With the Delta variant, we estimate R0 to be between 5 and 8, which means we need between 80% and 88% of the population to be immune.

Right now, we have approximately 45% of Utahns fully vaccinated, according to the Utah Department of Health. Again, it’s reasonable to assume that somewhere between 30% and 60% of the unvaccinated population has gotten COVID-19. That means roughly 60% to 75% of the population has immunity — enough to reach herd immunity levels with the old version of the virus but not the new one.

Essentially, we’ll likely see Delta variant case growth until a larger percentage of Utahns either get vaccinated or infected.

What are breakthrough cases like?

The good news is that breakthrough cases tend to be more mild. According to an analysis of over 11,000 COVID-19 cases in Michigan earlier in the year, hospitalizations were 96% lower in the vaccinated group than in the unvaccinated group — exceptionally good news!

That also matches data from the COVID Symptom Study, which asks users to track their COVID symptoms through an app. They found “fewer symptoms reported over a shorter period of time by those who had already had a jab” than those who haven’t been vaccinated. And those symptoms were relatively mild: headache, runny nose and sneezing were the top three.

There’s also the promising study from a Rhode Island prison system, which tested 2,380 inmates and staff who had been partially or fully vaccinated, and found 27 infections — 17 among the fully vaccinated group. All 27 of the cases found, though, were asymptomatic. Hurray!

The abundance of mild cases among the fully vaccinated has led some scientists to argue that we should redefine “breakthrough” cases. Their point is that some large percentage of these positive tests are actually only infecting people in their upper respiratory tract, where the primary immune system that reigns is mucosal immunity, not the blood-based immunity that the vaccines create. In other words, it’s not that these infections are “breaking through” the protection built by the vaccine, they just never reach it at all. It’s an interesting argument.

What about those who do have severe cases?

And yet, it’s definitely worth noting that there are many people who do have real breakthrough cases, and end up hospitalized or even dead as a result.

In Utah, there have been 198 breakthrough hospitalizations and seven deaths. While the number hospitalized is just 0.01% of those who have been vaccinated in Utah, that doesn’t make it any easier for those people or their families.

A study released Wednesday in Israel tried to find common characteristics between these breakthrough hospitalizations. Researchers took data from 17 of the 26 public hospitals in Israel and surveyed 152 breakthrough hospitalizations — about half of those in the country. They found that men made up 70% of the cases and that the hospitalized patients were on average 71 years old. Nearly all of them had comorbidities: 71% had hypertension, 48% had diabetes, 32% had heart failure and 40% were immunocompromised in some way before being infected.

We know that was the case during the pandemic — generally, the elderly and people with comorbidities were much more likely to be hospitalized. But those numbers greatly exceed even the skew we saw earlier with unvaccinated people — the average Utahn hospitalized for COVID-19, for example, was 54. In other words, it’s generally those who were at highest risk of hospitalization before who need to worry most about breakthrough infections.

To wit, in the Israel study, there were six people who were hospitalized with breakthrough infections of COVID-19 without comorbidities, and all six have had what the study called “good outcomes.” (The Michigan study, for what it’s worth, found that the difference in death for vaccinated vs. unvaccinated people, once hospitalized, wasn’t statistically significant.)

The disappointing news is that we haven’t discovered an obvious personal characteristic that ties together all of the breakthrough cases. So far, there’s no genetic link, or amount of initial virus load, or anything like that, that explains why some people get breakthrough cases and some don’t.

Those who are vaccinated but have multiple comorbidities may still want to be relatively cautious as we deal with the influx of the Delta variant. Those who are vaccinated with relatively low-risk profiles may still want to be on the lookout for mild infections — and get tested if necessary — to prevent further spread.

And for those who aren’t vaccinated? Change that, please. You’re letting the rest of us down.

Andy Larsen, one of The Salt Lake Tribune’s Jazz beat writers, doubles as a data columnist. You can reach him at alarsen@sltrib.com.