Avian Flu Diary: CDC: Serology Results On Missouri H5 Patient's Contacts (Updated)

Missouri - Credit Wikipedia

Updated: I've added the link and excerpts from the CDC Serology report.

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During today's CDC teleconference we learned the results of the long-awaited serology testing of the close contacts of Missouri's only confirmed H5N1 case. Due to recent changes to the surface proteins of the virus, a new test had to be devised, which caused several weeks delay.

While we should get a more detailed update later today, we now know that none of the 6 symptomatic Health Care Workers who were in contact with the index patient developed antibodies, meaning there is no evidence they were infected.

ANDA household contact who became ill at the same time as the index case did test positive on one of three tests (microneutralization Inhibition) - which, while not conclusive - is highly suggestive that they were infected with the H5 virus as well.

The timing of their illness, however, suggests a shared common exposure to the same source, rather than human-to-human transmission. Exactly what that source might be has not been determined, although it was likely from an animal or animal product exposure.

The reality is we may never know how this household was exposed. But at least there is no evidence of human-to-human transmission.

CDC Report on Missouri H5N1 Serology Testing

This spotlight was posted on October 24, 2024.

Topline Summary of Findings

In August 2024, healthcare providers in Missouri had a patient who had gastrointestinal symptoms with a history of chronic respiratory illness. The person was hospitalized and tested for multiple respiratory pathogens, and was positive for influenza A. They recovered from their illness and were discharged. Follow-up surveillance testing was conducted at the Missouri Department of Health and Senior Services Public Health Laboratory using polymerase chain reaction (PCR) that is part of routine national influenza surveillance. Testing found that the patient tested presumptive positive for avian influenza A(H5) ("H5 bird flu"). There was no history of exposure to infected animals or humans. The specimen was forwarded to CDC as part of routine protocol. CDC confirmed the specimen as H5N1 bird flu and this was reported as a case on September 6, 2024.

Missouri state and local officials performed a lengthy retrospective investigation of everyone who came into close contact with the patient and identified seven people (6 health care workers and one family member) who had symptoms that warranted additional investigation given their potential exposure to this patient. CDC, in support of this investigation, conducted a series of tests on blood specimens from 6 of these people – and the original patient – to help identify signs of previous infection or exposure to H5N1 well after full resolution of their symptoms. This was not possible for the others, whose suggestive symptoms had completely resolved at the time of interview. None of the results of this extensive testing and investigation support that human-to-human spread occurred. The health care workers who were tested showed no signs of previous H5N1 infection. They were all "sero-negative." This finding rules out person-to-person spread between the MO case patient and any of health care workers tested. The Missouri case and a household contact both had some evidence – though inconsistent – which suggested exposure to – or a previous infection with – H5N1 using these serologic tests.

To date, human-to-human spread of H5 bird flu has not been identified in the United States. CDC believes the immediate risk to the general public from H5N1 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.

Following these tests, CDC continues to assess that the risk that someone who has not had contact with an infected animal will become infected with H5N1 remains low.

(SNIP)

Conclusion of Missouri Investigation:

The 5 health care workers were definitively seronegative against H5 in all assays, which means they were not infected with H5. This finding rules out person-to-person spread between the index MO case patient and any of five health care workers. Symptoms reported by these contacts were not caused by their exposure to this patient.

Testing results of the sera from the MO case patient and their household contact were similar: both showed evidence of an antibody immune response to H5 in only one assay (that detects H5 neutralizing antibodies), but not on the other serologic assays used to detect infection. The weak immune signal suggests that it is possible that both of these people may have been exposed to H5 bird flu despite the fact that they did not meet accepted thresholds for seropositivity.

These similar immunologic results coupled with the epidemiologic data that these two individuals had identical symptom onset dates support a single common exposure to bird flu rather than person-to-person spread within the household. Intensive epidemiologic investigation has not identified an exposure to an animal or animal product exposure to explain these possible infections, and these serologic data cannot further elucidate the exposure leading to these possible infections.
End Note:

Historical World Health Organization serological case definition using single serum

Single convalescent serum, collected at ≥ 21 days after symptom onset (or last exposure to infected animals/persons), with a microneutralization antibody titer ≥ 1:40 to an A(H5) virus,

AND either