Peer review of: SARS-CoV-2 is transmitted via contact and via the air between ferrets (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
The manuscript submission by Sander Herfst and colleagues reports on the direct and airborne 
transmission of SARS-CoV-2 in a ferret model of COVID-19. The authors demonstrate in this animal 
model that virus transmission is higher by direct contact versus an airborne route of transmission. 
Their findings support infection prevention measures, such as social distancing, to limit the public 
health impact. There are several minor concerns that should be addressed prior to acceptance for 
publication. 
 
Item 1. The data presented in supplementary figure 2 should be presented as figure 2 as infectious 
virus titers are more relevant for public health risk assessment. The manuscript should also discuss 
any observations of signs of morbidity. 
 
Item 2. Additional details should be provided in the Methods section pertaining to the quality of the 
virus stock, including mycoplasma status and genome copy numbers. Were ferrets positive for enteric 
coronavirus? The type of swab used for nasal samples should be indicated, i.e, pediatric or adult 
flocked swab, etc... Why were nasal virus loads assessed by swab as opposed to nasal wash? 
 
Item 3. The transmitted viruses should be deep sequenced to examine potential adaptive signatures 
or quasi species expansion within the transmitted viral population. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
In the manuscript “SARS-CoV-2 is transmitted via contact and via the air between ferrets”, the 
authors examine contact and airborne transmission of SARS-CoV-2 in 4 sets of animals intranasally 
inoculated with the virus. The authors demonstrate that the virus is efficient in transmission to all 
direct (4/4) and aerosol (3/4) contacts. In contrast to another report on this topic (ref 21), the 
authors detect infectious virus in the airborne recipient animals. This observation provides an 
important advancement to the field - that not only can airborne SARS-CoV-2 exposed animals 
seroconvert, but they become infected and shed significant amounts of virus for multiple days. These 
data provide a tractable model to ask important questions about intervention strategies to limit 
airborne transmission of SARS-CoV-2. There are a few details and modifications to the figures that the 
authors should consider in order to strengthen the conclusions, as well as provide important insight to 
compare transmission studies of SARS-CoV-2 that may emerge from multiple groups. 
 
1. Please provide more details regarding the transmission setup, specifically the air flow rate in the 
cages, the room temperature/humidity, and the exposure duration of the direct and aerosol contacts. 
The last point is to clarify for this reviewer whether the exposure was for 21 days as loosely implied in 
the materials and method or 14 days as is standard for other transmission studies performed by this 
group. In addition, the authors may want to use these metrics, which are known to influence 
transmission of influenza viruses as a discussion point to contrast their results to those presented in 
ref 21 (Kim et al Cell Host and Microbe 2020). 
2. It seems clear that 3 out of 4 airborne contacts shed virus in the upper respiratory tract (table 1). 
Viral titer data should be presented in Figure S1 or in the main text to accompany RNA titers, rather 
than a TCID50eq quantification. Especially given that the authors seem to have this data already and 
additional BSL3 experiments would not need to be performed. 
3. Determination of antibody titers, presented in figure 2, does not seem to include endpoint titration. 

The authors reference Ref 31 for a description of the ELISA methodology, but this manuscript (Okba et 
al Emerging Infectious Disease 2020) does not describe an ELISA protocol where serial dilutions of 
sera are used for endpoint quantification. The authors should consider doing a dilution series of the 
antibody to obtain the EC50 of antibody binding to Spike RBD. A neutralization would be preferred but 
to minimize additional BSL3 work, a sera dilution would acceptable. Data on the day 0 serum should 
be included for each animal as well. 
4. Please provide details on the swabs used for the nasal collection, as this is not common for 
collection of viruses from the ferret upper respiratory tract.