ABSTRACT
Non-pharmaceutical interventions (NPIs) to contain the SARS-CoV-2 pandemic drastically reduced human-to-human interactions, decreasing the circulation of other respiratory viruses as well. As a consequence, influenza virus circulation - normally responsible for 3-5 million hospitalizations per year globally - was significantly reduced. With downscaling the NPI countermeasures, there is a concern for increased influenza disease, particularly in individuals suffering from post-acute effects of SARS-CoV-2 infection. To investigate this possibility, we performed a sequential influenza H1N1 infection 4 weeks after an initial SARS-CoV-2 infection in the ferret model. Upon H1N1 infection, ferrets that were previously infected with SARS-CoV-2 showed an increased tendency to develop clinical symptoms compared to the control H1N1 infected animals. Histopathological analysis indicated only a slight increase for type II pneumocyte hyperplasia and bronchitis. The effects of the sequential infection thus appeared minor. However, ferrets were infected with B.1.351-SARS-CoV-2, the beta variant of concern, which replicated poorly in our model. The histopathology of the respiratory organs was mostly resolved 4 weeks after SARS-CoV-2 infection, with only reminiscent histopathological features in the upper respiratory tract. Nevertheless, SARS-CoV-2 specific cellular and humoral responses were observed, confirming an established infection. Thus, there may likely be a SARS-CoV-2 variant-dependent effect on the severity of disease upon a sequential influenza infection as we observed mild effects upon a mild infection. It, however, remains to be determined what the impact is of more virulent SARS-CoV-2 variants. ImportanceDuring the COVID-19 pandemic, the use of face masks, social distancing and isolation were not only effective in decreasing the circulation of SARS-CoV-2, but also in reducing other respiratory viruses such as influenza. With less restrictions, influenza is slowly returning. In the meantime, people still suffering from long-COVID, could be more vulnerable to an influenza virus infection and develop more severe influenza disease. This study provides directions to the effect of a previous SARS-CoV-2 exposure on influenza disease severity in the ferret model. This model is highly valuable to test sequential infections under controlled settings for translation to humans. We could not induce clear long-term COVID-19 effects as SARS-CoV-2 infection in ferrets was mild. However, we still observed a slight increase in influenza disease severity compared to ferrets that had not encountered SARS-CoV-2 before. It may therefore be advisable to include long-COVID patients as a risk group for influenza vaccination.
ABSTRACT
Improving COVID-19 intervention strategies partly relies on animal models to study SARS-CoV-2 disease and immunity. In our pursuit to establish a model for severe COVID-19, we inoculated young and adult male ferrets intranasally or intratracheally with SARS-CoV-2. Intranasal inoculation established an infection in all ferrets, with viral dissemination into the brain and gut. Upon intratracheal inoculation only adult ferrets became infected. However, neither inoculation route induced observable COVID-19 symptoms. Despite this, a persistent inflammation in the nose was prominent in especially young ferrets and follicular hyperplasia in the bronchi developed 21 days post infection. These effects -if sustained- might resemble long-COVID. Respiratory and systemic cellular responses and antibody responses were induced only in animals with an established infection. We conclude that intranasally-infected ferrets resemble asymptomatic COVID-19 and possibly aspects of long-COVID. Combined with the increasing portfolio to measure adaptive immunity, ferrets are a relevant model for SARS-CoV-2 vaccine research.