There may be confusion today about what CDC's new mask guidance means about the protection of vaccination. There are multiple levels of protection, and it’s important to know which level is up for discussion. Quick review. 1/6
1. Any infection. If you are vaccinated, would your body fight off the virus so that you would never even test positive or become infectious? This scenario is what original CDC guidance allowing vaxxed people to unmask was banking on being most common. 2/…
2 Asymptomatic or presymptomatic infection and infectiousness. Say you get infected and would test positive but you have no symptoms. Could you transmit? We don’t rly know how common this is. Masks have particular value here because most people would not know they are infected 3/
3. Symptomatic and infectiousness. Say you are vaccinated, get infected, would test positive, and you have symptoms. You are likely infectious. Masks are important here as source control, but more importantly you must get tested and isolate at home. 4/
New topic, but relevant: Allowing vaxxed people to unmask may have had the side effect of everyone unmasking, vaxxed or not. Today’s mask guidance may also be intended to get more unvaccinated people to mask up again. 5/
Vaccines continue to be great at preventing severe illness, but delta may be making scenarios 2 and 3 somewhat more common. That possibility, the precautionary principle, getting unvaxxed to mask and rising hospitalizations all likely factor in to updated mask guidance. 6/6
In CDC's press conference just now, Dr. Walensky emphasizing scenario 3 as motivation for the new guidance.

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More from @cmyeaton

21 Jul
Time to return to indoor mask policies in states that are surging. It happened sooner than I expected, but when hospitalization trends look like this, something has to change.
Yes, it's perhaps unfair to vaccinated people, but with no way to differentiate a policy for all is the only practical way.
We also need to ramp up testing (again) and carefully prepare for school reopening. Mitigation measures must remain in schools: masks, ventilation, access to testing.
Read 5 tweets
14 Apr
One in a million will not be the final estimate for the unconfirmed J&J events and I think we should be cautious in citing it. The incidence will surely be rare, but @C_R_Watson and I wrote about biases in estimating mortality last Feb, and similar issues are in play here 1/
@C_R_Watson First, the numerator. Now that awareness has been raised, more cases may be reported. 2/…
Second, the denominator. All of the cases in US have been in women ages 18-48. It is reasonable to ask whether the denominator should reflect that. The expert committee will consider this. 3/
Read 5 tweets
29 Mar
Cases are on the rise in many states. How worried am I? I am concerned, but not as much as I would have been 6 months ago with these trends. Short thread. 1/
Many states are doing quite well. I would like to see cases fall below 20 cases per 100,000 per day as a first goal and 10 as a second goal (and then the lower the better). By that measure, we are doing ok: 35 states are at or below 20 and 12 are below 10. 2/
Yet some states are resurging. MI and NJ are at ~50 cases per 100,000 per day and hosp rising too. Risk in those states is high, and leaders should intervene by closing high risk settings and accelerating vaccination. Fed govt could help by sending extra vax coming online. 3/
Read 7 tweets
4 Feb
So, how are we doing with covid? Nationally, reported incidence has fallen from around 76 cases per 100,000 population per day to about 43. Better! But not yet good. Thresholds are contentious, but I think 20 as a first goal and 10 as a second goal are reasonable to start. 1/
Nationally, those thresholds correspond to approx. 66,000 and 33,000 daily cases, respectively. Right now, we’re at ~140,000 cases reported each day on average, so we need to more than halve incidence to get to Goal 1. 2/
What do 20 and 10 cases per 100,000 per day get you, practically? We will still be masking and distancing, for sure. In VT, where reported incidence is currently about 21, in a group of 10 people there is about a 10% chance someone has covid (caveat ahead) 3/
Read 7 tweets
1 Feb
Fantastic development. This test is already authorized for home use without a prescription, including in children and people without symptoms. Increasing supply critical to allow people to easily access tests.
The test was authorized in Dec but supplies are limited. The Biden Admin is investing in expanding manufacturing capacity. It will take a while (months, probably) for that to result in more supply available to consumers, but we'll need tests for a long time so still useful.
The ~$30 price point is a little high to keep a few in the bathroom cabinet, but it will be a nice option. Looks like the Federal government will keep some of the supply, will be interesting to see how they are deployed.…
Read 4 tweets
21 Jan
I could not be more thrilled that the Biden-Harris Administration has announced an intention to create a National Center for Epidemic Forecasting and Outbreak Analytics @dylanbgeorge 1/…
@dylanbgeorge This idea has a long history, with dozens of brilliant scientists, policymakers and innovators behind the wheel, including many who roll up their sleeves every day to make sure our leaders have what they need to fight outbreaks effectively @dylanbgeorge 2/…
But they shouldn’t have to self-organize or do this on a volunteer basis. We need a standing capability that can advance the science by creating and improve epidemic models and outbreak analytics… @dylanbgeorge 3/…
Read 6 tweets

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