Study evaluates efficacy of a candidate SARS-CoV-2 subunit vaccine to protect endangered black-footed-ferrets

By Dr. Chinta SidharthanOct 7 2022Reviewed by Aimee Molineux

In a recent study published in Viruses, researchers assessed the safety, efficacy, and immunogenicity of a candidate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) subunit vaccine in black-footed ferrets (BFF). They investigated the susceptibility of these endangered North American mammals to the virus.

Background

Mustela nigripes, or black-footed ferrets, are highly endangered North American mammals belonging to the family Mustelidae. Diseases have wiped out all but a few of the last remaining BFF population. The surviving BFFs have been raised in captivity at several facilities in North America in an attempt to restore breeding populations and improve their survival in the wild.

The recent coronavirus disease 2019 (COVID-19) pandemic revealed that domestic ferrets and related species of minks are susceptible to SARS-CoV-2, as are some other captive animals in zoos.

Given the susceptibility of closely related species to SARS-CoV-2, added precautions such as physical distancing, ultraviolet air filtration, and personal protective equipment (PPE) have been enforced at the BFF breeding facilities to prevent viral exposure. However, such measures are expensive and not sustainable over long periods and hinder the breeding program.

The development of SARS-CoV-2 vaccines for BFFs could effectively protect the existing populations from COVID-19 and limit the transmission of the virus between the animals and their human caretakers.

About the study

In the present study, researchers injected a SARS-CoV-2 S1 subunit vaccine mixed with the adjuvant alum into 15 one- to four-year-old male BFFs. The control group comprising nine similarly aged male BFFs received only the diluent and the adjuvant. Fecal samples of all the animals were tested before the immunization to rule out enteric coronavirus infection.

Booster vaccines and sham injections were given to the test and control groups, respectively, three weeks after the first injection. The vaccine doses were 50 µg, based on an earlier SARS-CoV-1 study using Syrian hamsters. The animals were observed daily for signs of morbidity.

Enzyme-linked immunosorbent assays (ELISA) were carried out on serum samples collected from anesthetized BFFs at the time of vaccination, time of booster dose, and two to three weeks and 12 weeks from boost. The serological tests were performed to test antibody titers to the SARS-CoV-2 S1 protein.

Transgenic mice expressing angiotensin-converting enzyme 2 (ACE-2) receptors were intraperitoneally administered the sera from the vaccinated and control BFFs to assess if the antibodies elicited by the vaccine in the BFFs were protective against SARS-CoV-2.

The mice were then intranasally inoculated with SARS-CoV-2 24 hours after serum transfer and observed for signs of morbidity and weight loss. They were subsequently euthanized, and tissue samples from the lungs, brain, nasal turbinate, spleen, liver, and intestines were collected for histopathological observations.

A set of six BFFs, comprising vaccinated and unvaccinated animals, were also virally challenged with SARS-CoV-2. These animals were euthanized post-viral challenge and necropsied for viral titer and histopathology tests.

Results

The results reported no adverse reactions to the vaccinations in all 24 BFFs. The serological tests using ELISA revealed a 60-fold increase in antibody titers after the first vaccination and a 150-fold increase after the booster vaccination compared to pre-vaccination levels. However, the mean titers decreased to 28-fold in all animals after 12 weeks.

The mice that received serum from vaccinated and unvaccinated BFFs developed an unexpected acute reaction to an antigen in the ferret serum, compromising the serum transfer and viral challenge study.

The viral-challenged BFFs, despite substantial viral shedding and SARS-CoV-2 replication in the upper respiratory tracts, did not exhibit any pathological lesions or other clinical manifestations of SARS-CoV-2. The younger of the two vaccinated viral-challenged BFFs exhibited detectable neutralizing antibodies and did not shed any virus after exposure, while the older BFF had no detectable antibodies, indicating that age was an important predictor of antibody titer.

Though the study showed that BFFs were not susceptible to SARS-CoV-2, they exhibited viral shedding after experimental infection, suggesting a potential for viral transmission between the animals and humans. The decline in humoral response in 12 weeks also suggests the need for higher vaccine dosages.

Conclusions

To summarize, the study investigated the efficacy, safety, and immunogenicity of a candidate SARS-CoV-2 S1 subunit vaccine in captive black-footed ferrets, to ensure these highly endangered mammals are protected against COVID-19.

The results indicated that the BFFs had no adverse reactions to the vaccines, and significant antibody responses were elicited to the immunization. The absence of clinical manifestations of COVID-19 in BFFs and the absence of SARS-CoV-2 infections in their primary prey suggest that wild BFF populations are not at risk.

However, given the close interactions between captive animals and human caretakers, the United States Fish and Wildlife Service has vaccinated 60% of the captive BFFs at the breeding facilities.