I don't have a concrete technical answer to this (regarding CDC's precise definition--they have a lot of pages on V-safe as how to participate in it, but no other published results besides those slides, it seems), but note that the same presentation puts anaphylaxis in a separate category (there were 6 cases in the same sample).
As far as the (higher) reactogencity of Covid-19 mRNA vaccines this has been noted in the scientific press before. If you want some vignettes, Science obliges with an article from Nov, based on the clinical trials...
Ian Haydon, who received the highest dose of the Moderna vaccine in its first human trial, knows what that’s like. (He received 250 micrograms, but partly in response to reactions like his, the company chose to take forward a lower dose of 100 micrograms.)
Twelve hours after receiving his second injection in May, Haydon got chills as well as “headache, muscle ache, fatigue, nausea,” and had a fever of 39.6°C. He went to urgent care, and later vomited and fainted before the symptoms receded, roughly 24 hours after they started, he says.
But Haydon says his experience was “a small price to pay” for the possibility of returning to normal life. “For me, this was a rough day. But if you compare it to what COVID can do, I think it really pales in comparison.”
Luke Hutchison, a Massachusetts Institute of Technology–educated computational biologist, volunteered for a trial of Moderna’s COVID-19 vaccine. After he got the second injection, his arm immediately swelled up to the size of a “goose egg,” Hutchison says. He can’t be sure he got the vaccine and not a placebo, but within a few hours, the healthy then-43-year-old was beset by bone and muscle aches and a 38.9°C fever that felt, he says, “unbearable.” “I started shaking. I had cold and hot rushes,” he says. “I was sitting by the phone all night long thinking: ‘Should I call 911?’”
Hutchison’s symptoms resolved after 12 hours. But, he says, “Nobody prepared me for the severity of this.”
Also from the CDC presentation, which is based on a sample of some 272,000 people vaccinated, that's about a 1% incidence of the "Health Impact Events". (Actually, the V-safe data was reported for just 112,807 participants; since V-safe is a phone app for self-reporting one cannot expect this to match other monitoring systems except approximately; relative to the V-safe reporting sample, the incidence was around 3%.) In the clinical trials that preceded FDA authorization, the incidence of various kinds of temporary side-effects (e.g. headache, fatigue) was actually higher than that, but for "severe" ones that "prevent[ed] daily activity", the incidence was around 2% in the clinical trials:
most people will escape “severe” side effects, defined as those that prevent daily activity. Fewer than 2% of recipients of the Pfizer and Moderna vaccines developed severe fevers of 39°C to 40°C. But if the companies win regulatory approvals, they’re aiming to supply vaccine to 35 million people globally by the end of December. If 2% experienced severe fever, that would be 700,000 people.
Other transient side effects would likely affect even more people. The independent board that conducted the interim analysis of Moderna’s huge trial found that severe side effects included fatigue in 9.7% of participants, muscle pain in 8.9%, joint pain in 5.2%, and headache in 4.5%. For the Pfizer/BioNTech vaccine, the numbers were lower: Severe side effects included fatigue (3.8%) and headache (2%).
That’s a higher rate of severe reactions than people may be accustomed to. “This is higher reactogenicity than is ordinarily seen with most flu vaccines, even the high-dose ones,” says Arnold Monto, an epidemiologist at the University of Michigan School of Public Health.
Adds Drew Weissman, an immunologist at the University of Pennsylvania whose work contributed to both vaccines: “The companies just have to warn people: ‘This is what you need to expect. Take Tylenol and suck it up for a day.’”
The CDC has a detailed page with reactogenicity results from the Moderna's clinical trial. From that page, in 65 and older people, "chills" were reported by a whopping 30% of participants on the 2nd dose (non-placebo; for placebo it was 4%), but "Chills – Grade 3: prevented daily activity and required medical intervention" only by 0.8% (non-placebo; for placebo it was "< 0.1%"; in raw numbers: 27 vs 2 cases of "grade 3 chills" on vaccine vs placebo on the 2nd dose). For 18-64 y.o., the proportion who reported any chills was even higher on the 2nd dose, 48.3% (5001 cases) vs 730 (6.4%) on placebo. And for "grade 3" chills, the number in that younger group was 151 (1.5%) on the 2nd dose, vs 14 (0.1%) on placebo.
They also have a similar "grade 3" distinction for the other 4 categories of side-effects reported: "Fatigue, Myalgia, Arthralgia", "Nausea/Vomiting", headache, and fever; you can read the detailed stats there... The numbers for "grade 3" fever (39.0 – 40.0°C) were roughly comparable with those for (grade 3) chills. The "grade 3" myalgias were however about an order of magnitude higher (10%) in the 18-64 y.o., but only about half of that in 65 and older. ("Any" myalgia was reported by around half the subjects on the 2nd dose, non-placebo.)
They have an analogous page for Pfizer's vaccine too, from clinical trial data. I didn't delve into the numbers for this... but at a glance it uses a similar breakdown by grade "Mild: does not interfere with activity; moderate: some interference with activity; severe: prevents daily activity; Grade 4: emergency room visit or hospitalization", but the Pfizer page reports more detailed numbers for each grade.