The Consequences of Reporting Lags in Interpreting Measles Case "Declines"

Measles cases in the US are up to 378. Based on the CDC's measles case graph you might think the epidemic is about to vanish. Not so - lags matter.

Measles cases in the US are now up to 378. The CDC reports once a week on Fridays including a graph of the outbreak. A visitor to the CDC’s measles outbreak page might think cases are about to vanish. Not so. This brief post explains why reporting lags matter. It should not take long to read, but it may change the way you interpret reports on measles case “declines”. I would also hope the CDC considers revisiting the way it communicates incomplete data to send a clearer message to the public.

Measles case snapshot from the CDC webpage… it looks like things are vanishing.. not so fast.

I encourage folks to visit the CDC measles outbreak webpage today, March 21st. If you do, you will see a bar chart with measles cases. Zoom in to the right. This is what you see. 41 cases in the week of March 9th (through March 15) and 5 cases in the week of March 16th (through 22nd). Visually it appears the epidemic is almost done. Or is it?

Measles cases the week of March 16 (through March 22)… 5 cases… for now

The website has an appropriate disclaimer: "case counts are preliminary and subject to change." But the website doesn't have a different shading or indication that the case counts for recent weeks may be VERY different than final counts once case reports continue to come in. So let's go back in time? Oh wait, we can't easily do that on this page. But we can if we downloaded last week's data, expecting that counts would change (& yes, I did expect that and yes, I downloaded the data from last week).

When you compare data reported last week to data reported this week, it's not just this week's cases that are newly reported but many cases from weeks in the past. The following bar graph distinguishes measles cases reported as of Friday March 14th vs measles cases newly reported as of Friday March 21st.

Newly reported measles cases this week go back many weeks… and change a narrative of what is happening in the national epidemic.

What does this mean? Simply put it means that apparent declines in new measles incidence should be interpreted as *apparent declines due to reporting lags*. Rather than going down from 58 to 47 cases the week of March 2, there was an increase from 66 to 70. So cases were not going down in early March, they were actually going up. And this could keep happening.

Cases appear to be going down, but are actually going up. Be aware.

The reality is that under-reporting, lags in testing, and poor messaging from HHS leadership means that measles represents a national threat to unvaccinated individuals, including those who never had the chance to make a choice because they are too young, immunocompromised, or have a legitimate medical exemption.

HHS leadership must step up and do what it takes to get this right and stop the spread of a preventable diseases. Measles is serious. Approximately 1 in 5 cases have been hospitalized - needlessly. Vaccines are safe & effective. Get vaccinated now if you are not vaccinated already. And spread the word.