Reply to Bernstein, Atmar, and Hoft

TO THE EDITOR—Thank you for the opportunity to respond to the Letter to the Editor submitted by Drs Bernstein, Atmar, and Hoft regarding the endpoints that have been used in our influenza challenge studies. Although we agree that it is important to use high-quality endpoints in human challenge models, we do not agree with their arguments against the use of “mild-to-moderate influenza disease” (MMID) as one of several endpoints in a challenge study [1–3]. This endpoint has been used as the primary endpoint in initial validation studies of new influenza challenge models including this influenza A/H3N2 virus as it encompasses both virologic and clinical measures of disease, giving us an objective measure to characterize the performance of a model in which every participant is exposed to infectious influenza after it is purposefully administered intranasally.

We recognize that all influenza symptoms are nonspecific and are shared with many other respiratory viruses. These symptoms can include fever, but it has been clearly demonstrated that many influenza infections do not result in fever, either in challenge studies [4] or from natural infection [5–7], especially in young adults. Therefore, in evaluating the performance of a challenge virus, we prefer to include a broadly defined list of symptoms to better understand the full spectrum of illness associated with infection. Each participant is assessed daily; if a reasonable explanation for the cause of a symptom other than influenza is found, it is documented and not deemed as meeting MMID criteria as the symptoms must be considered influenza-related. In this setting where participants are carefully screened on admission and tested for 21 different respiratory pathogens daily before and after influenza administration, the symptoms assessed are broad but are known symptoms of influenza [8, 9] and in the absence of other infections, in this context are most plausibly due to influenza.

The authors of the letter point out that several of the H3N2 challenge participants had influenza symptoms without documented viral shedding, indicating that the symptoms were not influenza related. We counter that the detection of viral shedding is not necessary to indicate that a person has been infected with influenza. It is common for individuals to develop influenza infections, including medically attended influenza, without a positive virologic test due to the limitations of sample collection that include: the location and kinetics of the replicating virus, anatomy of the person, variation in sampling techniques (ie, nasal wash, nasopharyngeal wash, nasal swab), and the diagnostic test being performed. This is recognized in many influenza challenge studies [10], including the one referenced in the letter [11] that relied on the isolation of influenza virus and/or a 4-fold or greater increase in serum hemagglutination inhibition (HAI) antibody titers to define influenza infection, recognizing that there are instances when influenza infection occurs without the detection of virus.

Human challenge studies have been used for decades [12] and have allowed for great advances in the development of influenza countermeasures [13]. In using these models for evaluation of novel vaccines or therapeutics it is important to choose endpoints that are apt for the goal of the study. The MMID endpoint is only one of many endpoints we have developed in these challenge models. In all of our challenge studies we include analysis of virologic, immunologic, and clinical endpoints to assess the severity of illness, including the number of days of shedding, the number of days of symptoms, the number of symptoms, and FLU-PRO scores, in addition to many laboratory-based measures such as antibody responses and transcriptomics [14]. All of these endpoints together allow for a more complete picture of the disease to be assessed and any of them could be considered as endpoints in phase II trials. Through careful development of novel influenza challenge models with endpoints like MMID, these validated models can be applied in various ways and at various stages in the development of novel broadly protective influenza vaccines to assess efficacy as well as breadth of protection, and they will play a key role in the development of the next generation of influenza vaccines and countermeasures.

Note

Potential conflicts of interest. The authors: No reported conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest.