Could you refresh the definitions of IFR, RO, HIT, and 1-1/R0. I bet 99.35% of us can define all of them.
Thanks.
That should be CAN’T!
Sure thing!
IFR is the Infection Fatality Rate. It’s the rate at which people die when contracting a particular disease, irrespective of how many cases are caught or missed. You may have seen the CFR or Case Fatality Rate published before (usually by the media when they want to pump up the fear). For the CFR, you take the total number of deaths (e.g. 314,629 confirmed in the US so far) and divide by the total number of cases (17,394,314), or ~1.8%. However, the CFR is subject to biases, particularly when you have a lot of missed cases. Essentially you’ll pretty much always catch deaths (the numerator), but you’ll miss a lot of cases (the denominator), making the percent higher than it should be.
IFR corrects for this mismatch between that measure and reality. It does so by looking at things like antibody (serology) studies compared to diagnosed cases. It looks like how many cases are likely to be missed due to asymptomatic or mildly symptomatic people not being tested. Apply some statistical modeling to all that data, spit out a result. In the case of SARS-CoV-2, that comes out to 0.65%, which is far lower than the ~1.8% CFR. Now you see why the media only ever talks about the Case Fatality Rate!
R0 is the basic reproductive number. In essence, if you have one person infected in a population where everyone is vulnerable, how many people - on average - will that person infect? An R0 of 1 means each person who’s infected infects one other. This is the minimum sustainable number for a disease. An R0 under 1 means the disease is spreading so slowly that the number of infected persons is dwindling over time. An R0 of 3 means if I’m infected, I’ll infect 3 others. Now 4 have it. They’ll each infect 3 others. Now 13 have it (me + 3 I infected + 9 they infected). Then 40, etc. SARS-CoV-2 has an R0 of 2.5.
HIT is the Herd Immunity Threshold. When you recover from an infection, you typically develop immunity to it. So as more people in a population become infected, recover, and develop immunity, the number of vulnerable hosts drops. This means R0 (which is based on everyone being vulnerable) is no longer valid because some are immune. The Re (”effective reproductive rate”) or Rt change over time based on people becoming immune either through infection or vaccination. The Herd Immunity Threshold is the point where Re dips below 1. Remember, that’s the point where new infections will begin to dwindle as each person infects fewer than one other on average. Important to note this is not a wall where the virus vanishes. Rather, it’s the beginning of the end for the virus.
1-1/R0 is the formula to calculate HIT. You simply take 1-1/R0 (R0 being 2.5 for SARS-CoV-2, so 1-1/2.5). 1/2.5 is 0.4 and 1-0.4 is 0.6 or 60%. So once 60% of the population becomes immune (either through infection or vaccination), Re will drop below 1 and new infections will dwindle over time.
Hope that helps!