#18,198
In addition to announcing the 5th & 6th human H5 infections from the Weld Colorado poultry outbreak, last night's CDC's weekly avian flu update contained details on a small seroprevalence study looking at dairy workers in Michigan, and partial sequencing of the virus in Colorado.
Some excerpts from yesterday's update include:
Reporting the preliminary results of the Michigan-led seroprevalence investigation.
CDC analyzed sera (blood) collected from people who were exposed to dairy cattle infected with highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b viruses causing outbreaks among animals in the United States. These blood samples were collected as part of a type of study called a seroprevalence study. Seroprevalence studies test people’s blood for antibodies (an immune response) specific to a pathogen of interest, in this case HPAI A(H5N1). These studies can tell us whether someone has been previously infected.
- Blood samples were collected in June 2024 from 35 people who work on dairies in Michigan with herds that were confirmed positive for HPAI A(H5N1) virus.
- Study participants were from multiple counties and had different roles on affected farms, but most worked with sick cows directly and fewer than half reported using masks or goggles.
- These samples were tested for antibodies against an avian influenza A(H5N1) clade 2.3.4.4b virus and a seasonal influenza virus (control virus) to measure antibodies.
- Specimens were tested by two methods: microneutralization and hemagglutination inhibition (HI) assays.
- None of these 35 people showed neutralizing or HI antibodies (a sign of prior infection) specific to avian influenza A(H5N1) virus.
- Many of the people had neutralizing antibodies to seasonal flu.
- The detection of antibodies to seasonal flu suggests that, not unexpectedly, participants in the study had been previously infected or vaccinated with seasonal influenza viruses and were able to generate an immune response.
- The lack of antibodies to avian influenza A(H5N1) virus suggests these people were not previously infected with an avian influenza A(H5N1) influenza virus.
- These data are consistent with other data demonstrating the seroprevalence to HPAI A(H5N1), even among workers with known exposures, is low.
- Additional data are necessary to fully understand the occupational risks of exposure to the currently circulating avian influenza A(H5N1) viruses.
- The Michigan Department of Health and Human Services is continuing the study and working with CDC to analyze the data and prepare a manuscript for peer-review publication once complete
The CDC also confirms what we first learned about earlier this week GISAID: The Plot Thickens; that it is the bovine B3.13 genotype that has spilled over into poultry, and then into humans, in Weld County, Colorado.
- Posting the sequence of the influenza virus genome from a positive specimen collected from one patient in Colorado participating in an A(H5N1) infected poultry farm depopulation (A/Colorado/109/2024) to GISAID (EPI_ISL_19263923) and GenBank (PQ032835). CDC sequenced the influenza virus genome, confirming the neuraminidase (the N in the subtype) is an N1 and the virus is a HPAI A(H5N1) virus from clade 2.3.4.4b. An analysis of the sequence indicates:
- This virus is genotype B3.13 clade 2.3.4.4b HPAI A(H5N1) with each individual gene segment closely related to viruses detected in recent poultry outbreaks and infected dairy cattle herds made available from USDA. Among human virus sequences, A/Colorado/109/2024 is most similar to the genome of the human case in Michigan.
- The sequence maintains primarily avian genetic characteristics and lacks changes that would make the virus better adapted to infect or spread among humans.
- The sequence contains a change at PB2 M631L – which is the same marker of mammalian adaptation identified in >99% dairy cow sequences and that was also identified in the first Michigan human case.
- The genome does NOT have the PB2 E627K change that was seen in the virus from the case in Texas, but not subsequent human viruses. (That change is known to be associated with viral adaptation to mammalian hosts and has been found before in people and other mammals, but with no evidence of onward spread among people.)
- Furthermore, there are no markers known to be associated with influenza antiviral resistance found in the virus sequences from the patient's specimen.
- This is the only virus that CDC has been able to fully sequence out of the cases in Colorado so far. Partial gene segments were obtained from two of the other confirmed human cases associated with the poultry farm depopulation. Only internal genes (not HA and NA) generated sufficient data for analysis to date. This data confirmed identical gene sequences among the CO cases based on available data.
- The HA sequence of the Colorado poultry worker virus is very closely related (only 2 or 4 amino acid changes) to two existing HPAI A(H5) candidate vaccine viruses [IDCDC-RG78A (A/American Wigeon/South Carolina/22-000345-001/2021-like) and IDCDC-RG71A (A/Astrakhan/3212/2020-like), respectively] that are already available to manufacturers, and which could be used to make vaccine if needed.
- There are no changes to the virus that would suggest the risk to human health has increased.
- Overall, the genetic analysis of the HPAI A(H5N1) virus in Colorado supports CDC's conclusion that the human health risk currently remains low.
While there is certainly some heartening news in these reports, it is also fair to say that the goalposts have recently been moved.
Had anyone dared predict last March - that in 4 months time - we'd have roughly 160 cattle herds infected with H5N1 across 13 states (an underestimate), that we'd be battling large mid-summer outbreaks of H5N1 in commercial poultry, and that we'd have chalked up 10 human cases in the U.S., they'd have been called an alarmist.
A reminder that we shouldn't confuse `increasingly commonplace' with `normal'.
