CDC has just concluded a press briefing and the big news is there is no big news. In fact there was hardly any small news. The major questions have been identified — how transmissible, what is the epidemic curve, are there more cases in the US, are there subtle genetic differences in the US and Mexican versions to account for the apparent difference in clinical and epidemiological features, etc. — but answering them will take longer.
Meanwhile, no new cases have been identified in the US, but CDC in collaboration with state and local health departments and the academic and medical sectors are working doing aggressive case finding. Critical care clinicians are being asked to look carefully at acute pulmonary distress and atypical pneumonia cases and suspect cases of flu-like illness in recent travelers to Mexico or the affected areas in Texas and California are continuing. No positives have yet turned up but everyone expects more confirmed cases to be recognized. The general scenario is for a primary health care provider to do a rapid flu antigen test which can tell quickly if a patient has influenza A or B but not what kind. Influenza A specimens then go to public health laboratories in the states to be subtyped (whether it is H1N1, H3N2, H5N1). The state labs now can do these fairly sophisticated analyses because of training consequent to the concern over H5N1 (“bird flu”). This is a good example of how infrastructure development has served a virtal public health purpose. If the specimen is untypable by the state lab it is sent on to the CDC reference lab in Atlanta for a closer look. CDC says that even these untypable specimens can usually be typed by CDC but sometimes something new appears, and that is the case with this virus. After sequencing it was clear it was genetically different than anything in their libraries. This required construction of new PCR primers.
There was no new information about the situation in Mexico, except to say that “the situation is serious. We in CDEC are worried.” Some, but not all, of the specimens from severely ill or deceased cases sent from Mexico is the new swine flu virus (7 of 14 specimens). While there is still no explanation for why the disease appears more severe, this is one of the top questions to be answered. Is it some difference in the information (looking at severely ill versus routine surveillance of outpatients), some difference in the virus (while the viruses are said to be “genetically identical” this is true only in the parts that have so far been compared) or some co-factor (e.g., co-infection with another pathogen). Determining the epidemic curve in Mexico or anywhere else (the evolution of cases over time) for a viral syndrome that is very non-specific (lots of noise from other viruses that cause the same syndrome) and for which diagnostic tests are time consuming, specialized and often unreliable is inherently difficult. It requires painstaking, tedious and time consuming effort. Us of the scanty and spotty syndromic surveillance systems in the US so far does not indicate an unrecognized outbreak of mild disease but CDC and state health departments are looking hard.
It is very clear that CDC has given this the highest priority and it sounds to these ears that they have very competent and dedicated personnel devoted to it. But reliable science takes time, care and some patience, even in circumstances where urgency is high and patience in short supply. That’s just the way it is.
CDC has constructed a new and easier to navigate website for this: http://www.cdc.gov/swineflu
Perhaps the most important message is that this is a good time to move forward on strengthening the public health infrastructure and to get ready for the one thing we can be certain of in the days ahead: there will be more uncertainty.