Surface–Aerosol Stability and Pathogenicity of Diverse Middle East Respiratory Syndrome Coronavirus Strains, 2012–2018

Neeltje van Doremalen; Michael Letko; Robert J. Fischer; Trenton Bushmaker; Jonathan Schulz; Claude K. Yinda; Stephanie N. Seifert; Nam Joong Kim; Maged G. Hemida; Ghazi Kayali; Wan Beom Park; Ranawaka A.P.M. Perera; Azaibi Tamin; Natalie J. Thornburg; Suxiang Tong; Krista Queen; Maria D. van Kerkhove; Young Ki Choi; Myoung-don Oh; Abdullah M. Assiri; Malik Peiris; Susan I. Gerber; Vincent J. Munster

Disclosures

Emerging Infectious Diseases. 2021;27(12):3052-3062. 

In This Article

Discussion

The ongoing MERS-CoV endemic in the Middle East and subsequent discovery of the virus in camel herds across Africa has resulted in a wealth of publicly available genetic data for various viral strains and isolates. In this study, we assessed several of these isolates for viral phenotypes related to public health in an attempt to better inform public health policy making with regards to MERS-CoV and other human coronaviruses that cause respiratory diseases, such as SARS-CoV-2.

Because nosocomial spread is at the center of MERS-CoV outbreaks, we assessed the stability of the virus on various surface material types commonly found in hospitals (polypropylene plastic and stainless steel), as well as materials that had potential antiviral and known antimicrobial properties (silver and copper).[27,28] Our experiments were performed at environmental conditions similar to those in hospitals, in which there is high risk for human-to-human, nosocomial transmission. Regardless of the surface material tested, strain C/KSA/13 was the least stable over time and was below detectable levels by 24 hours (Figures 2, 4). This strain had the lowest starting titer in these experiments, which might explain this difference in stability. In addition, our C/KSA/13 stock contains 2 nonsynonymous mutations in the viral structural proteins, spike and matrix, not found in our other strains, which might also play a role in this difference, either directly or indirectly (Table). These findings warrant further studies on how specific MERS-CoV polymorphisms in structural proteins affect viral growth.

As shown by Doremalen et al.,[21] all virus strains tested had notably reduced stability on copper and silver surfaces (Figure 2, panel A). Copper has been shown to also have antiviral properties against influenza A(H1N1) virus and SARS-CoV-2.[29–31] The exact antiviral mechanism for copper is still unclear, but might be related to formation of hydroxyl radicals by copper ions when in aqueous solution.[31] Silver-based nanoparticles have been shown to be antiviral for HIV-1,[32] herpes simplex virus 2,[33] hepatitis B virus,[34] respiratory syncytial virus,[35] and monkeypox virus.[36] Taking advantage of the antiviral properties of copper and silver might help decrease nosocomial transmission. Both silver and copper can be used for coating medical tools[37] and commonly touched items, such as bed rails, door handles, and intravenous poles.[38] These findings appear to be more broadly applicable for other coronaviruses because we observed similar results for SARS-CoV-2 (Figure 2).[21] Further research should be invested in determining coronavirus susceptibility to metal ion inactivation.

MERS-CoV transmission might occur through aerosols and fomites,[39] although the role of each route is not known. Transmission often occurs in hospitals; thus, aerosol-generating medical procedures might play a major role.[40] MERS-CoV transmission has occurred over distances of >6 feet,[41] and evidence of MERS-CoV on surfaces and in air in hospitals has been found.[39] Studies have suggested that a hospital air-handling system might have contributed to nosocomial spread during the 2015 MERS-CoV outbreak in South Korea,[14,39] and our group has shown that the virus can remain viable suspended in air for ≤10 min.[26] We tested aerosol stability of viral isolates and observed that all viruses remained viable for a minimum of 180 min with an ≈10-fold reduction in viral titer observed on average within the collected aerosols (Figure 2, panel B).

Although we did not observe major differences in this study, strain stability is an useful phenotype to continue monitoring because mutations in viral capsid proteins have been shown to enhance environmental stability of bacteriophages, dengue virus, and transmissible gastroenteritis virus.[42–44] Because MERS-CoV isolates contain polymorphisms throughout the entire viral genome, including the structural proteins that form virions, mutations might arise that influence overall virus particle stability. C/KSA/13, which showed reduced stability on surfaces in our experiments, contains polymorphisms in open reading frame 1b, the spike glycoprotein, and the virion matrix protein in comparison to the other strains tested. Recent studies have further demonstrated the influence of various external factors on environmental stability for SARS-CoV-2, including experimental ambient conditions and matrix in which the virus is suspended.[45,46] Our experiments were performed in standard, indoor laboratory settings and with virus suspended in culture media, which enabled us to observe intrinsic differences determined solely at the viral level. Tracking and assessing the stability of coronavirus strains will improve our understanding of coronavirus variant spread.

We tested viral replication kinetics in Vero E6 cells and primary HAE cells (Figure 3). All viruses replicated to similar titers on Vero E6 cells by 72 hours. However, KSA/15 and C/KSA/13 had higher titers than EMC/12 by 48 hpi. Albeit the difference is not significant, C/BF/15 has a lower viral titer than EMC/12 at 48 hpi and 72 hpi. These results are consistent with those of a previous study, which showed that C/BF/15 has impaired replication.[18] In primary HAE cultures, all camel-derived viral isolates had reduced replication kinetics compared with that for EMC/12 (Figure 3, panel B). More studies are needed with these camel-derived isolates to determine whether their differences in replication kinetics results from a comparison with EMC12, which has well-described tissue culture adaptations, or to see if MERS-CoV might adapt in humans after transmission from camels. Sequence analysis of the viral variants did not identify any obvious mutation patterns in any single viral protein that would explain the differences in replication kinetics. Thus, we speculate that these differences are the result of cumulative effects across ≥1 types of genetic variation.

We have shown that MERS-CoV replicates in type I and II pneumocytes in the lower respiratory tract of an animal model.[20] Although disease progression after infection with this virus does not involve the central nervous system in humans, this small animal model is suitable for vaccine candidate testing, using animal survival or viral-induced death as a binary readout for vaccine efficacy. MERS-CoV C/BF/15 contains a deletion in open reading frame 4b, which has been shown in a similar mouse model to result in impaired suppression of the host interferon response and increased type I and type III interferon signalling.[18] Taken together, these results pave the way for testing MERS-CoV vaccine candidates for broadly neutralizing potential in this animal model.[20,47]

Our results with MERS-CoV C/KSA/13 suggest there might be a potential tradeoff between environmental surface stability and replication kinetics. This tradeoff was observed for a camel-derived isolate, and we did not observe similar phenotypic relationships for the other strains tested (Figures 2, 3). Future research efforts with camel-derived viruses and more closely related human-derived viruses could show whether adaptations are likely to occur after zoonosis. Our previous viral stability results with SARS-CoV-2 and the findings of this study with MERS-CoV suggest copper should be incorporated more in hospital settings, particularly in materials in areas of high contact between hospital workers and MERS patients, such as door handles, bed rails, and medical tools.[21] Overall, we observed a range of stability, replication, and pathogenesis phenotypes between different MERS-CoV isolates, underscoring the need for continued surveillance of this virus and other coronaviruses, including SARS-CoV-2.

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