Transmission and Protection against Reinfection in the Ferret Model with the SARS-CoV-2 USA-WA1/2020 Reference Isolate

Abstract

SARS-CoV-2 has initiated a global pandemic and several vaccines have now received emergency use authorization. Using the reference strain SARS-CoV-2 USA-WA1/2020, we evaluated modes of transmission and the ability of prior infection or vaccine-induced immunity to protect against infection in ferrets. Ferrets were semi-permissive to infection with the USA-WA1/2020 isolate. When transmission was assessed via the detection of vRNA at multiple timepoints, direct contact transmission was efficient to 3/3 and 3/4 contact animals in two respective studies, while respiratory droplet transmission was poor to only 1/4 contact animals. To determine if previously infected ferrets were protected against re-infection, ferrets were re-challenged 28 or 56 days post-infection. Following viral challenge, no infectious virus was recovered in nasal wash samples. In addition, levels of vRNA in the nasal wash were several orders of magnitude lower than during primary infection, and vRNA was rapidly cleared. To determine if intramuscular vaccination protected ferrets, ferrets were vaccinated using a prime-boost strategy with the S-protein receptor-binding domain formulated with an oil-in-water adjuvant. Upon viral challenge, none of the mock or vaccinated animals were protected against infection, and there were no significant differences in vRNA or infectious virus titers in the nasal wash. Combined these studies demonstrate that in ferrets direct contact is the predominant mode of transmission of the USA-WA1/2020 isolate and immunity to SARS-CoV-2 is maintained for at least 56 days. Our studies also indicate protection of the upper respiratory tract against SARS-CoV-2 will require vaccine strategies that mimic natural infection or induce site-specific immunity. Importance: The SARS-CoV-2 USA-WA1/2020 strain is a CDC reference strain used by multiple research laboratories. Here, we show the predominant mode of transmission of this isolate in ferrets is by direct contact. We further demonstrate ferrets are protected against re-infection for at least 56 days even when levels of neutralizing antibodies are low or undetectable. Last, we show that when ferrets were vaccinated by the intramuscular route to induce antibodies against SARS-CoV-2, ferrets remain susceptible to infection of the upper respiratory tract. Collectively, these studies suggest protection of the upper respiratory tract will require vaccine approaches that mimic natural infection.

SARS-CoV-2 and Influenza A Virus Coinfections in Ferrets

“…Ferrets are susceptible to experimental infection of SARS-CoV-2, and the infection is dose-dependent, virus shedding is undetectable when the infectious dose is low ( 53 ). However, ferrets are semipermissive to natural infection by direct contact, and no infection was detected by droplet transmission ( 54 ). In contrast, IAV single infection (CA/09 H1N1) did not cause any body weight loss in hamsters, while IAV and SARS-CoV-2 coinfection caused hamsters to lose 9.6% of their body weight at 4 d postinfection ( 24 ).…”

Animal Models for COVID-19: Hamsters, Mouse, Ferret, Mink, Tree Shrew, and Non-human Primates

“…No obvious lesions were observed in other tissue, except for multifocal inflammatory cell infiltration was observed in the portal areas of liver ( Ryan et al, 2021 ). Ferrets are fully protected from acute lung pathology following re-challenge of the virus, which indicates naturally acquired immunity helps ferrets against reinfection ( Patel et al, 2021 ; Ryan et al, 2021 ).…”

Animal models for SARS‐CoV‐2 infection and pathology

“…Direct contact with SARS-CoV-2-infected ferrets can induce the transmission of virus to the naïve ferrets, which occurs as early as 2 dpi. 161,171,172 Transmission via respiratory droplets and/or aerosols is efficient between ferrets over more than 1 m distance, 173 but shows a delayed infection for 3-7 days 172 compared to the direct contact. However, in one study, the respiratory droplet shows a low efficiency with only 25% of infection between naïve ferrets.…”

“…However, in one study, the respiratory droplet shows a low efficiency with only 25% of infection between naïve ferrets. 171 The pattern of clinical features, viral replication, pathology, and immune response of the infected naïve ferrets resemble those of inoculated ferrets. 161,172 To date, the ferrets are well used as models to measure the countermeasures against SARS-CoV-2 transmission.…”

“…162 The induced immune response protects ferrets from adverse effects when rechallenged the SARS-CoV-2, including lower viral replication, no remarkable lesion in the upper and lower respiratory tracts, and no significant bronchopneumonia. 162,163,171 However, lack of ferretspecific immunological reagents severely impedes the further investigation of immunological mechanisms in this animal model. Some new technologies such as singlecell RNA (scRNA) sequencing, 176 have been employed to resolve this dilemma.…”