COVID Q & A – Transmission & Prevention


Scientifically-informed, data-driven answers to your burning questions about the coronavirus pandemic  

Post #1: Transmission and Prevention

Welcome to a new Naturally Speaking blog series on COVID-19. This first post about Transmission and Prevention comes in two flavours:

  1. Short and sweet – bite size summary
  2. Hungry for more? Look no further! This version includes a bit more detail and links to further resources.

If we don’t answer all your most pressing questions, please feel free to ask them in the Comments section below – we’ll do our best to respond. We’ll also aim to provide any updates as advice and knowledge evolves.

Disclaimer: Some of the guidance refers specifically to the situation in the UK, although most of the content is relevant regardless of where you hail from.

*Last updated on 13 May, 2020.

Q. Can people pass on the virus BEFORE showing any symptoms?

A. Yes. It is not yet clear how often this happens, but some people who show few/no symptoms (asymptomatic) can be infectious with the virus (they can spread it to others). As many as 60% of all infections could be asymptomatic. Additionally, some people can infect others before they show any disease signs. We don’t fully know how long this ‘pre-clinical infectious period’ can last. What we do know, however, is that even with this element of pre-symptomatic transmission, enough people do show signs when they’re sick that it can be possible to control the virus based on targeting visible or symptomatic cases. This has been demonstrated in places like South Korea, Singapore etc., if rapid testing and quarantining is enforced.

In general the studies on this pre-symptomatic virus transmission are pretty variable. Major uncertainties include how people manifest and report symptoms – these could differ by demographics, infectious dose, underlying health conditions etc. – so it will be very difficult to broadly apply estimates across different populations. Here are a few case studies that provide some information:

  • On the Princess Diamond cruise ship (small sample size, older population), an estimated 41% were asymptomatic upon detection, falling to 31% by the end of the observation period.
  • Another clinical study preprint showed 44% of patients were infectious 2-3 days before symptoms, and another study of almost 500 patients in China indicated about 13% pre-symptomatic transmission (this newspaper article comments on the study).
  • Another study from a small number of patients (18) from Wuhan found one case wherein the virus was detected without the patient showing any signs of illness. However, the highest viral loads (when transmission is most likely) were detected just after patients started to show symptoms.

Generally, undocumented infection is a huge obstacle in stemming the spread of infection, and several studies, such as this one, estimate how under-detection contributed to coronavirus spread early in the pandemic. Despite these challenges, one town in Italy, Vo, successfully showed that large-scale testing and tracing was able to control an outbreak.

Q. Should we be worried about take-away food? How about food from supermarkets?

A. As a general rule, standing in line with lots of people is not advisable! If take-away means a busy queue, it would be better to consider delivery, or to sharpen your own culinary skills. While there are likely some minor risks from food prepared by others, including handling the outer packaging, the risks of contracting the virus from touching surfaces decline over time and can be mitigated by washing hands and not touching your face. This article in the Washington Post nicely describes how several factors all have to come together for infection to occur, and we have quite a bit of control of several of those factors, enabling individuals to cut the chain of transmission. Over the past several weeks, we’ve seen positive actions being taken by supermarkets to minimize contact among customers, both in the queues which are now more spread out, and inside the stores by limiting the number of customers who can enter at a time. Consider wearing gloves when shopping, and always wash your hands before entering and after leaving public areas. Generally, highest transmission risks are from household contacts and travelling with infected people (e.g. public transportation).

Infectious material can last on surfaces up to 72 hours. However, the half life of the virus is only 6 hours (the time it takes for half of the virus on a surface to die; about half of the surviving virus would die in the subsequent 6 hour-period, and so on). The highest risks are thus from being directly in contact with others who are infectious (coughing, shaking hands, hugging etc). There are lots of key workers doing deep cleaning of doctors’ surgeries and hospital wards, who are much more likely to be in contact with infectious material – we should be deeply grateful to these workers – this is a huge responsibility!

Q. Does the mode of transmission affect the severity of disease?

A. We’re not aware of any research on this yet. For many viruses the severity of disease is dose-dependent: the larger the viral dose, the more severe the disease. But this is not always true, so it’s difficult to extrapolate. In Post 3 we discuss some of the factors that are currently thought to affect the severity of the disease.

Q. Why the 7 or 14 day isolation?

A. If you are sick with suspected COVID-19, UK government recommendations are to wait 7 days until you recover and can interact safely with others (cease being infectious). If you have been in contact with someone who is sick, particularly another member of the household, you should self-isolate for 14 days, as it can take this long for symptoms to appear. If someone develops symptoms during this period, they should continue to self-isolate for 7 days from the start of the symptoms. 

Q. Is it worth buying N95 face masks? Do they really work?

A. Yes, they work. At the beginning of the pandemic, personal protective equipment (PPE), including N95 masks, was in short supply, so it was important for it go to frontline health workers who urgently needed it, i.e. those putting themselves at risk through their work, but who ultimately protect many more people. For this reason they were not being recommended for the general public. N95 masks should be fitted by someone trained to do so, and are not for use by kids or folk with beards! While other types of face masks aren’t as effective at protecting people from infection, that can help reduce the spread of the virus via aerosol droplets from those who are infected (including asymptomatic cases who don’t know that they are infected!). Masks might also help remind people not to touch their faces. DELVE, an independent scientific group convened by the Royal Society, published a valuable report on May 4th that summarises the evidence about the potential value of face masks in tackling the pandemic.  

Q. Any idea whatsoever on timings of length of quarantine(s)?

A. Likely 3 months or longer. We can’t know at present. Governments will aim to keep this as short as possible, but if compliance is low and the epidemic worsens, stricter enforcement will be likely, and for longer periods. Beyond 3 months is very unclear for many reasons.

Critically, the effects of severe restrictions won’t even start to be seen for approximately 2 weeks or more, so we need to be both compliant and patient. We discuss the ‘exit strategy’ in more detail in Post 4.

An important modelling study informing the government response makes for fairly grim reading. In the absence of any intervention (unrealistic) over half a million deaths in the UK are predicted over the next 2 years. They explored 2 very different approaches – “mitigation”, to slow but not necessarily stop spread, vs “suppression” to reverse the epidemic. Even in the best case, “mitigation” overwhelms the health system 8 times over.

Suppression (basically lockdown) could still overextend the health system. But models indicate that if maintained over the next 3 months, these measures could limit the number of deaths to <10,000. The Catch 22 is that these measures need maintaining until we have better drugs and/or a vaccine. Realistically this means 18 months or longer. In response to this, they examined the effects of an adaptive strategy, implementing ‘suppression’ for 5 months then monitoring cases and switching ‘suppression’ on and off (2 month/1 month) for the next 18 months, which they considered the best bet.

We have seen from China that distancing measures substantially reduced their epidemic (and gave the rest of the world valuable time to prepare). But studies also warn that once those measures relax resurgence is likely, and this is a major concern going forward. On a positive note, over the foreseeable future, many other factors will come into play – e.g. better drugs, serological testing, preparedness/ capacity building to increase testing, robust contact tracing tools and hopefully a vaccine (see Post 2).

Q. Why are some countries making people stay indoors? Shouldn’t going for a walk be low/no risk if you stay 2m apart?

A. Generally, keeping 2m distance outdoors should be fine and is working in some countries. But if people don’t comply with social distancing guidance, the disease will escalate and more severe measures will be inevitable – beyond a tipping point, any opportunity to reduce transmission will be crucial.

Keeping roughly 2m apart seems to be working in some countries with less severe distancing measures (e.g. South Korea, Singapore, Taiwan) if the disease is not escalating. But enforcing that is hard. Especially if people don’t take risks seriously. Where limited distancing is working is in countries that have experienced SARS, so the public were already alert and responsive, and health services experienced and prepared (arguably governments also more trusted). So people act responsibly.

In situations where severe lockdowns have been applied, cases were rising so rapidly every single reduced opportunity for transmission makes a difference – these places could not cope with more cases. But distancing measures do not work overnight; at this point the situation is usually desperate/ perceived to be escalating that way. Generally any effects of these measures on new cases won’t be seen for 2 weeks (see chart 7, i.e. if you are healthy but infected today – before lockdown – you won’t become ill until next week), and reductions in deaths won’t be seen for 3+ weeks (given the duration of illness and critical care needed for those who do get very sick). So these really are drastic measures, but even in this situation some people need to work, so residual risks of transmission are hard to avoid.

Q. Can my cat or dog become infected and/or sick from the COVID-19 virus?

A. Over the past weeks, questions have emerged about the potential of pets getting sick from coronavirus. While there have not yet been many studies, it does appear that cats and ferrets, and to lesser extent dogs, are susceptible to infection. Of these, cats may also show signs of clinical disease, although few details are currently available about what specific signs are most common. At the end of April, cases of COVID-19 were reported on two mink farms in the Netherlands, where they showed respiratory signs. However, cases in domestic animals appear to be quite rare. While animal-to-animal transmission has been demonstrated in laboratory settings, most animal infections are likely from close contact with infected people.

Q. Can pets transmit the COVID-19 virus to people?

A. It is very unlikely that pets can transmit the COVID-19 virus to people. The position of the British Veterinary Association (BVA) is that the ‘risk of transmission from pets is infinitesimally small’, and the World Organisation for Animal Health (OIE) states that the vast majority of transmission is from human to human. Current recommendations by the BVA and the European Advisory Board on Cat Diseases are for cats to be kept indoors for households where people are sick with COVID-19, although ideally cared for by a household member without symptoms.

Feature image is original artwork by PhD candidate Chiara Crestani, ©2020.

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