During the first week of classes at the University of Illinois, campus testing identified 165 cases of Covid on a total of 22,296 tests.
For comparison, in the first week of the spring semester (Jan 25 - 31), we had a total of 128 cases with 63,208 tests. We're finding more cases than at the comparable point last spring despite testing far fewer people. Whether that's due to Delta or to reduced restrictions and more potential super spreader events (e.g., football games with unrestricted attendance, massive new student orientations with lots of yelling, open bars, etc.) is hard to know.
And, as noted in my post earlier today, we're missing the vast majority of breakthrough cases because we are not doing surveillance testing of vaccinated students. If the vaccines have 60% efficacy against infection and the exposure risks are comparable for vaccinated and unvaccinated students (an assumption that might not be justified), we could have up to 3x as many breakthrough cases that have gone undetected. That is, had we tested everyone on campus, we could well have found a much higher number of cases (perhaps hundreds). Any breakthrough cases that that we didn't detect (because we're not looking) potentially could infect others. Even if people with breakthrough cases do not spread covid as easily as unvaccinated people with Covid, that's still a lot infected people who come into close contact with many other people.
We don't definitive evidence of vaccine efficacy against infection in this population, and we lack information from the university about the total numbers of people being tested, the number of each subgroup of people on campus, or the numbers of vaccinated and unvaccinated people in each group being tested. Without that information, we can only rely on guestimates about efficacy as well as infection rates. But, even if we ignore the possibility of large numbers of undetected breakthrough cases, the first week numbers aren't great - they look worse than at the start of the spring semester. If there are a sizable number of undetected breakthrough cases, as seems inevitable, we won't know how much spread we're seeing on campus and we won't be able to disrupt chains of transmission. I hope breakthrough cases aren't as infectious, but if they are even somewhat infectious, we're risking undetected exponential growth.