We know that respiratory viruses can be spread through droplets, occasionally aerosols, and contact routes (see Figure 1). But what is the relative importance of these transmission routes for the SARS-CoV-2 virus, which causes COVID-19? A new pre-print paper published yesterday provides evidence that the stability of the SARS-CoV-2 coronavirus is broadly comparable to the ‘original’ SARS coronavirus (SARS-CoV-1) on dry surfaces and in aerosols. This paper supports an important role for dry surface contamination and aerosols in the spread of SARS-CoV-2, and suggests that improved environmental persistence isn’t the key to the relative success of SARS-CoV-2 over SARS-CoV-1.

Figure 1: Transmission routes of respiratory viruses (from this review article).

The study team tested the ability of the SARS coronavirus (SARS-CoV-1) and the SARS-CoV-2 coronavirus (which causes COVID-19) to survive on a range of dry surfaces (copper, steel, plastic, and cardboard) and in aerosols generated in a drum. Virus survival was broadly comparable between SARS-CoV-1 and -2, with both viruses surviving considerably longer on steel and plastic surfaces than on copper, cardboard, or in aerosols (Figure 2). The half-life of SARS-CoV-2 on plastic was notably high, at around 16 or 17 hours. Even on copper, cardboard, and in aerosols, survival was measured in hours not minutes, and therefore could contribute to transmission.

Figure 2: Measured and modelled decay rates (A) and half lives (B) of SARS-CoV-1 and HCoV-19 (=SARS-CoV-2) in aerosols and on surfaces.Since the SARS-CoV-1 and -2 viruses are so similar, structurally speaking, it’s no surprise that they exhibit similar traits in terms of environmental persistence. However, it is important to note that both SARS-CoV-1 and -2 (and indeed the MERS coronavirus) have unusual environmental persistence characteristics compared with other enveloped viruses.

In 2016, I published a review of the evidence on the role of surface contamination in the spread of SARS, MERS, and influenza viruses. This seems to be made relevant again by the laboratory findings of this study. The key findings of the (now out of date!) review were that SARS and MERS coronaviruses can be shed into the environment, be transferred from contaminated surfaces to hands, and result in self-inoculation events. Mathematical and animal models, and intervention studies suggest that contact transmission may be more important than droplet transmission under some circumstances.

Whilst these principles are likely to be applicable to SARS-CoV-2, we have very limited evidence on the role of surface contamination in the spread of this virus. There was much discussion about widespread environmental contamination of the food market where COVID-19 was thought to have originated. This recently published article references this widespread contamination of the food market, but I can’t find the details of this published (like which surfaces were sampled, which method was used etc). And I haven’t seen much evidence (so far) of environmental contamination during investigations of COVID-19 clusters. However, I did come across this study, also on a pre-print server, which tested environmental and air contamination with SARS-CoV-2 in an infectious diseases unit of a Chinese hospital that was caring for patients with COVID-19. The rate of contamination of surfaces and air with SARS-CoV-2 was low: 1 (0.8%) of 130 surfaces and 1 (3.6%) of air samples, and the virus was detected through PCR so no guarantees about viability. However, the positive environmental sample was collected from a shared nursing station, illustrating the potential risk for transmission.

In the round, this evidence suggests that contamination of surfaces and aerosols are likely to play a role in the transmission of SARS-CoV-2. Therefore, national guidelines in the UK around using a disinfectant for environmental disinfection (not necessarily a sporicidal agent) and an FFP3 respiratory for aerosol generating procedures are spot on. However, the relative importance of droplet, aerosol, and contact spread for SARS-CoV-2 is unclear and likely to depend on the circumstances.