On Friday, the Centers for Disease Control and Prevention (CDC) reported that a person who lived with a Missouri resident infected with the H5N1 bird flu virus became ill on the same day as the initial patient. This raises concerns about the potential for human-to-human transmission of the virus, which would be a first in the United States.
CDC officials noted that, at this time, there is no epidemiological evidence to support person-to-person transmission of H5N1. They emphasized the need for further research. The timing of the illnesses, which occurred outside the typical flu season, has alarmed some experts. H5N1 is known to spread among close contacts, including household members.
Neither the initial patient nor the household contact had reported any exposure to the virus through animals or raw milk. The household contact, who exhibited gastrointestinal symptoms, was not tested for H5N1, so it is unclear if they were actually infected.
Details about both patients remain limited. The household contact’s illness was not previously reported by the CDC or Missouri health officials. During a Thursday news briefing, CDC officials had described the initial case as isolated and had not mentioned the household contact’s illness.
Missouri health officials reported on Thursday that all known contacts of the initial patient remained asymptomatic during the observation period. The CDC later acknowledged that information about the household contact’s illness should have been included in the briefing.
Criticism has emerged regarding the lack of disclosure. Jennifer Nuzzo, director of the Pandemic Center at Brown University School of Public Health, criticized the omission, stating that it was unacceptable to withhold such information.
The CDC is investigating whether both cases resulted from simultaneous exposure to H5N1, but experts caution that symptom onset timing alone cannot confirm exposure. Deepta Bhattacharya, an immunologist at the University of Arizona, noted that the timing of symptoms can vary widely.
The hospitalized patient with confirmed H5N1 was admitted on August 22. The household contact developed symptoms on the same day, but the case was not detected by routine flu surveillance because the contact was not hospitalized. Missouri health officials are considering antibody testing to check for H5N1 exposure.
Dr. Nirav Shah, CDC principal deputy director, stated that it is too early to detect antibodies in the hospitalized patient, a view contested by Dr. Bhattacharya, who believes enough time has passed to conduct such tests.
Other experts have expressed frustration over the limited information available regarding the investigation. Dr. Nahid Bhadelia, director of the Center on Emerging Infectious Diseases at Boston University, called for more details on the case investigation.
In addition to the household contact, a health worker who attended to the hospitalized patient also became ill but tested negative for flu. CDC scientists faced difficulties obtaining a complete genetic sequence of the virus from the hospitalized patient due to sample quality issues.
Since the H5N1 outbreak in dairy cattle was first reported in March, the virus has been detected in 203 herds across 14 states and 14 people. The virus has not been found on Missouri’s dairy farms, although commercial poultry flocks and wild birds have been affected. The first 13 human cases involved people who worked directly with infected poultry or cows.
The Missouri case was identified through routine flu surveillance. The patient, who had underlying medical conditions, was treated with the antiviral drug Tamiflu and has since recovered. Federal officials have not disclosed further details about the patient due to privacy concerns.
