That's a really good dataset (with a really clunky visualization, but hey!)
Indeed. There are typically above 3000 deaths/week due to Pneumonia and 200/week due to influenza at this time of year. We had a smallish spike in mid-late January, where for 3 consecutive weeks, deaths were above trend at nearly 4000 & 400 per week.
If you assume a 1% mortality rate for COVID, and you assume all deaths are due to pneumonia, and you assume the entire late-January spike is due to COVID, that could be hiding 300,000-400,000 US cases. (To be clear, those are all questionable assumptions! The mortality rate could easily be either lower or higher, a meaningful fraction of deaths might not show as pneumonia or influenza, and it is normal for there to be a moderate spike above trend at some point in the flu season)
That doesn't mean that there were or weren't a few hundred thousand cases in that time -- it just means that some significant number of cases could easily hide in "routine-looking" spikes in the graph.
This does mean, though, that it is unlikely that everybody who got "the really bad flu" that's been going around this year actually had COVID -- but, especially if you live in a place with lots of people who travel to China regularly (like I do!), it isn't unreasonable to wonder if that "really bad flu followed by a lower-respiratory-tract infection that tested negative for influenza" you got in late January/early February wasn't really COVID.
(Remember, though, that while the rapid influenza test has a pretty low false-positive rate, it's got about a 50% false negative rate -- so testing negative shouldn't lead you to assume it isn't influenza)
If you are in ICU with flu symptoms, they don't just shrug when the rapid influenza test comes back negative. There are reflex tests that can be run to confirm positive or negative.
Critical cases and fatal cases are way more likely to be detected for this reason -- but during the 3 weeks I was talking about, it was believed that there was no COVID in the US, except for folks evacuated from China to quarantine sites.
During that time unexplained pneumonia leading to death -- especially in an older individual -- would likely not have been thought to be COVID, and would just have gone on the books as pneumonia.
