Vaccine against Middle East respiratory syndrome coronavirus

In The Lancet Infectious Diseases, Kayvon Modjarrad and colleagues¹ reported results of the first in-human clinical trial of the GLS-5300 vaccine candidate against Middle East respiratory syndrome (MERS)coronavirus. The vaccine induced both humoral and cellular MERS coronavirus-specific immune responses. These data suggest that GLS-5300 has potential value in protecting humans from MERS coronavirus infections. However, who should be vaccinated?

Epidemiological surveys² concluded that camel contacts, health-care workers, and patient household contacts are high-risk groups. Therefore, they should be the target groups for the vaccine. However, there are many infection cases for which the source of infection could not be identified. The unpredictability of these infections makes it hard to prevent human infections through vaccination for postexposure prophylaxis.

For zoonotic disease prevention and control, one-sided disease prevention (either human or animal) is often inefficient. Severe acute respiratory syndrome (SARS) is a successful precedent for containment of emerging coronaviruses based on elimination of the primary reservoir. Although the pathogens of SARS and MERS are both coronaviruses, two major factors lead to different control results. First, although the number of human SARS cases is greater than for MERS, most were due to a so-called super-spreader strain. By contrast, all clades of MERS coronavirus are shared by camels and humans, indicating that MERS coronavirus can easily spill-over from camels to infect humans.³ Second, palm civet, the intermediate amplifying host of SARS coronavirus, is an exotic animal that could easily be controlled by banning wild animal trading. However, the intermediate host of MERS coronavirus (dromedary camel) is an important livestock for the Middle East, with key roles in transportation, food, and fabric (wool); thus, it is impossible to eliminate all camels.

To control this disease in camels two possibilities exist: mass slaughtering of infected animals or vaccination. Unfortunately, serological surveys showed a very high prevalence (up to 100%) of MERS coronavirus-neutralising antibodies in dromedary camels.⁴ Therefore, it seems impossible to eliminate this disease by mass slaughtering of positive animals. Vaccination of camels seems to be the only choice, but no licensed vaccine for camels is currently available, although several vaccines are in development.⁵ Additionally, GLS-5300 has been reported to also be immunogenic in camels.⁶ Camels are not used routinely in research and, for most laboratories, it would be hard to attain enough animals and work with them. Moreover, they are too big to study in most biosafety facilities. These limitations restrict vaccination tests in camels.

It is increasingly recognised that a One Health approach is needed for effective investigation, prevention, and control of emerging zoonotic diseases. In the context of emerging zoonoses, human and veterinary medicines must work together. The eradication of MERS coronavirus in dromedary camels is the primary condition for the control of this disease in the Arabian Peninsula. If the virus continues to circulate in camels, it might attain new mutations that enable human-to-human transmission, resulting in the generation of super-spreader strains. A comprehensive MERS prevention and control effort should focus not only on a human vaccine but also on camel vaccination.