Yesterday DemFromCT had another in his continuing series at DailyKos on Flu and You (Part VIII). He extended an earlier post (part II) on a critical piece of public health infrastructure, laboratory surveillance. One of the graphics is this chart of influenza positive tests reported to the CDC by the WHO/NREVSS collaborating laboratories:
What you see in this chart is a weekly record of what seasonal influenza types and subtypes are circulating in the community (influenza A/H1N1, A/H3N2, B; swine flu makes a late appearance, far right). Flu seasons differ on dominant subtypes, whether they are well matched to the vaccine, and which ones are circulating where and when. These data are important for health services demand and determining the composition of the next year’s vaccine.
When laboratories can’t match a virus against the seasonal varieties — when it isn’t “typable” — CDC looks to see if it is a “novel” virus. Their own laboratories can often type specimens that are “untypable” by less sophisticated labs, but sometimes this turns up a virus that is truly new. That’s what happened with swine flu (see our posts here and here for more details on how it turned up).
Data like the one in this chart don’t just happen. They are the product of a multicomponent surveillance system, in this case the part that collects information from 80 laboratories that are part of a network of state and some county public health laboratories and large academic medical centers integrated into a World Health Organization (WHO) network of Collaborating Laboratories; and another 70 labs part of CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS). NREVSS is collects information on other respiratory and enteric viruses, but has integrated its laboratories into the CDC influenza surveillance system.
At any one time many viruses circulate that can cause respiratory syndromes indistinguishable from influenza, but when flu season starts the proportion of specimens that test as flu A or B starts to rise. Each week the 150 or so participating labs send CDC the total number of specimens they tested and the proportion that are positive for influenza A or B. The identity of the other viruses may or may not be known, depending on the virus and the laboratory (NREVSS is a surveillance system for some of them). Because of the concern over avian influenza, most of the US 80 WHO Collaborating labs have now been trained to determine influenza A seasonal subtypes (H1, H3), information they report to CDC along with the patient’s age. Most of the other 70 NREVSS labs don’t have that ability. That’s why the chart shows many specimens as “untyped” (not the same as “untypable”). The data from these 150 laboratories form the basis for the weekly report of the total number of positive influenza tests, by virus type/subtype, and the percent of specimens testing positive for influenza. Some of the specimens from the WHO collaborating labs are also sent to a specialized CDC lab for gene sequencing, antiviral resistance testing and specific strain determination. Information from the laboratory surveillance is also used in some methods to estimate the number of excess deaths due to influenza.
We’ll discuss some other components of the influenza surveillance system in subsequent posts. The salient point DemFromCT has made so well is that the laboratory infrastructure is a critical part of the general public health system. Surveillance is one of those unsexy, silent parts of ;ublic health that doesn’t get much support or attention until you discover you really, really need the data.
If you don’t have the infrastructure by then, it’s too late.