Just under one-third of the coronavirus cases reported in Sitka’s since June are “breakthroughs.” State epidemiologists say that statistic is impressive, but not particularly unexpected given the large, vaccinated population. Unvaccinated individuals are far more vulnerable. The vaccine remains the best defense against the coronavirus (KCAW/Berett Wilber)

State health officials are still crunching the data from the latest outbreak of COVID-19 in Sitka and some other Alaskan communities, and haven’t drawn any firm conclusions from the apparently high rate of “vaccination breakthrough”  — that is, infections occurring in individuals who are fully vaccinated.

Nevertheless, they say the evidence is clear that vaccination is still the number one tool in the fight against COVID– and the math bears that out.

The place where math meets medical science is a bit squishy for those of us who nodded off in Statistics class. The hard numbers I remember, though. This is Sitka Fire Chief Craig Warren speaking to the Sitka Assembly on July 13.

“Since June 1 there have been 87 positive cases of COVID-19,” said Warren. “Twenty-one of those cases were breakthrough. Breakthrough cases defined as somebody who has complete vaccination plus two weeks. Last week alone, there were 60 new cases, 18 of those were vaccinated.”

Warren went on to add that this was not a failure of the vaccine, which was never expected to offer 100-percent protection. In fact, state health officials point to dramatic decreases in the infection rate statewide directly attributable to the vaccine.

Still, one-third seems like a lot of breakthrough cases, doesn’t it? There’s something about this proportion that makes it feel like the vaccine isn’t working — but that’s a misperception. Dr. Louisa Castrodale monitors infectious diseases in the state Section of Epidemiology. When I asked about this high number of breakthroughs, she said she was “impressed.”

“It’s been interesting this timeframe, you know, in the last two months or so when we’ve seen communities that have had sort of a real increase in activity, it’s been impressive, the proportion that have been vaccine breakthrough,” said Castrodale. “And I kind of wonder if that’s also related to the fact that when we had those big outbreaks, or we had those sort of real increases in community activity before it was kind of pre-vaccine, and so that opportunity didn’t present itself until we had a population that was vaccinated.”

As Castrodale observes, this might be another thing that’s creating the perception that the vaccine isn’t as effective as it should be: Sitka didn’t experience an outbreak this big prior to the vaccine. Nevertheless, the vaccines are working because there are many more unvaccinated people contracting COVID than vaccinated people. Breakthrough cases occur only among vaccinated people, which is why looking at them as a proportion of overall cases can be misleading.

Even for the math-averse, it’s not a difficult relationship to understand: In Alaska right now, about half the population falls into each group, unvaccinated and vaccinated. At the beginning of February this year, when vaccine distribution began ramping up, the state was seeing about 1,000 new cases of COVID per week among the unvaccinated, and only a handful of cases among the vaccinated. As more and more people were vaccinated and new infections began to decline into the spring and summer, there were still a “low and steady” handful of breakthroughs among the vaccinated every week (656 total breakthroughs February 1 – June 30, 2021).

Anna Frick does infection control for the state Section of Epidemiology. She explains that the state’s breakthrough rate seems like it’s growing, but only because the overall number of cases is dropping.

“Because the total number of cases is decreasing so much, the proportion of this bar that is made up of that small number of vaccine breakthrough cases is increasing, even though the rate is lower.”

In Sitka (and elsewhere) we’ve got two populations contracting COVID– two denominators, unvaccinated and vaccinated. Statistically, the 21 breakthroughs out of 87 new infections that Fire Chief Craig Warren was describing are not as significant as the fact that those 21 breakthroughs occurred among a vaccinated population of over 5,000 people. Sitka’s unvaccinated population of around 2,000 (not including children under 12) experienced three times as many cases in the same period. Breakthroughs will always happen, especially as the vaccinated population grows. “Low and steady,” as Anna Frick said. It’s not bad odds, and it remains a strong argument for vaccination.

Although the math is favorable, the virus has been spreading rapidly in Sitka recently. The more-contagious Delta variant is the likely culprit for Sitka’s surge, but it hasn’t been confirmed. The state testing laboratory is only now beginning to genetically sequence samples from the beginning of July. There are seven other variants that are being tracked that haven’t received nearly as much attention.

Dr. Joe McLaughlin is the head of the Section of Epidemiology. He says that among the variables at work in the pandemic, there is this:

“People are gathering more,” he said. “Fortunately people are spending most of their time outdoors gathering, which we know is associated with lower transmission. But as people gather indoors — and even outdoors — if they’re really in close proximity there is the risk of transmission there.”

Even if genetic sequencing proves that the Delta variant is prevalent in Sitka, McLaughlin and the rest of the team at the state Department of Health and Social Services say vaccination is the best defense.