Dis­ease mod­eler makes es­ti­mates, courts con­tro­versy


Last au­tumn, when Martin Meltzer cal­cu­lated that 1.4 mil­lion peo­ple might con­tract Ebola in West Africa, the world paid at­ten­tion.

This was, he said, a worst-case sce­nario. Meltzer is the most fa­mous dis­ease mod­eler for the pre-em­i­nent U.S. public health agency, the Cen­ters for Dis­ease Con­trol and Preven­tion. His es­ti­mate was pro­moted at high-level in­ter­na­tional meet­ings. It ral­lied na­tions to step up their ef­forts to fight the dis­ease.

But the es­ti­mate proved to be off. Way, way off. As in 65 times worse than what ended up hap­pen­ing.

Some were not sur­prised. Meltzer has many crit­ics who say he and his CDC col­leagues have a habit of will­fully ig­nor­ing the com­plex­i­ties of dis­ease out­breaks, re­sult­ing in es­ti­mates that over-dra­ma­tize how bad an out­break could get — es­ti­mates that may be skewed by pol­i­tics. They say Meltzer and com­pany also over­es­ti­mate how much vac­cine is needed and how ben­e­fi­cial it has been.

Overblown es­ti­mates can re­sult in un­nec­es­sary gov­ern­ment spend­ing, they say, and may fur­ther erode trust in an agency that re­cently has seen its ster­ling rep­u­ta­tion de­cline.

“Once we cry wolf, and our dire pre­dic­tions turn out not to be the case, peo­ple lose con­fi­dence in public health,” said Aaron King, a Univer- sity of Michigan re­searcher who in a re­cent jour­nal ar­ti­cle took Meltzer and oth­ers to task for mak­ing what he called avoid­able mis­takes.

Meltzer, 56, is un­bowed. “I am not sorry,” he said. He dis­misses his peers’ more com­pli­cated cal­cu­la­tions as out of touch with po­lit­i­cal ne­ces­si­ties.

What Meltzer does is not par­tic­u­larly glam­orous. He and oth­ers use math­e­mat­i­cal cal­cu­la­tions to try to pro­vide a more pre­cise pic­ture of a cer­tain sit­u­a­tion, or to pre­dict how the sit­u­a­tion will change. They write equa­tions on chalk­boards, have small meet­ings to de­bate which data to use, and sit at com­put­ers. Meltzer spends a lot of time with Ex­cel spread­sheets.

But mod­el­ers have be­come crit­i­cal in the world of in­fec­tious dis­eases.

Top CDC of­fi­cials came to Meltzer last sum­mer, when the epi­demic was spi­ral­ing out of con­trol and in­ter­na­tional health of­fi­cials were quickly try­ing to build a re­sponse. Meltzer was asked to pro­ject how bad things could get if noth­ing was done, as well as to es­ti­mate how stepped-up aid could bend the curve of the epi­demic.

Meltzer and his col­leagues cre­ated a spread­sheet tool that pro­jected un­in­ter­rupted ex­po­nen­tial growth in two coun­tries, Liberia and Sierra Leone.

His pre­dic­tion — pub­lished

last Septem­ber — warned that West Africa could be on track to see 500,000 to 1.4 mil­lion Ebola cases within a few months if the world sat on its hands and let the epi­demic blaze.

About 21,000 cases ma­te­ri­al­ized by mid-Jan­uary — a ter­ri­ble toll, to be sure, but also just a tiny frac­tion of the caseload Meltzer and his CDC col­leagues warned about.

Did Meltzer blow it? Many say no. He and his col­leagues clearly stated they were pro­vid­ing a worst-case sce­nario of how bad things could get. They also pre­dicted a far lower num­ber of cases if more help was sent — which al­ready was hap­pen­ing when the model es­ti­mates were re­leased.

But the worst-case fig­ures got the most at­ten­tion. The media fo­cused on them in head­lines. Health of­fi­cials high­lighted them in their push to get more money and man­power de­voted to the epi­demic. And in­ter­est­ingly, those are the num­bers health of­fi­cials de­scribe as the most suc­cess­ful part of Meltzer’s pre­dic­tion pa­per.

“I think it gal­va­nized coun­tries — and peo­ple — to put in more ef­fort” into fight­ing the epi­demic, said Dr. Keiji Fukuda, for­merly a col­league of Meltzer’s at CDC who is now as­sis­tant di­rec­tor-gen­eral of the World Health Or­ga­ni­za­tion.

CDC is sup­posed to pre­pare Amer­ica for the worst, so it makes sense for its mod­el­ers to ex­plore ex­treme sce­nar­ios. If Meltzer’s es­ti­mates push pol­i­cy­mak­ers to bol­ster public health de­fenses, it’s all to the greater good, some say.

But there are oth­ers who feel that the re­sult cor­rupts both science and pol­i­tics.

“Public health of­fi­cials are well aware that their sta­tis­tics get used — and mis­used — to jus­tify an in­crease in their fund­ing” or to bol­ster vac­ci­na­tion cam­paigns and other ef­forts, said Peter Doshi, as­sis­tant pro­fes­sor at the Univer­sity of Mary­land School of Phar­macy.

Mod­el­ing — so poorly un­der­stood by the public, the media, and even many peo­ple in public health — pro­vides an op­por­tu­nity to bend num­bers to sup­port goals, he ar­gued.

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