Lessons learned about viral transmission

It’s been a common catchcry in this current pandemic – whether it be testing, transmission or treatment – ‘we just don’t have enough evidence.’

However in an analysis piece in the BMJ, a group of UK researchers argue that from what we know already about COVID-19 and what we have proven in the past with other pandemic viruses – we can confidently recommend behavioural interventions that we know will reduce community transmission – in particular transmission in households with one affected member.

It has been the experience of countries such as the UK, with a high caseload of COVID-19 patients, that there have been high infection rates among health workers. This has been the result of both frequent contact with infected patients and exposure to a higher viral load, as sicker patients have higher viral loads.

Consequently, those health care workers dealing with COVID-19 patients in hospitals are now wearing full PPE. But most patients who test positive for COVID-19 don’t wind up in hospital. In the UK, like here, patients who test positive and have mild, if any, symptoms are told to go home and self-isolate, including isolating from household contacts.

But here’s where it gets tricky. It is not practicable that all the household contacts wear full PPE. And not all households can truly isolate one member, so which measures are the most important? And can we really prevent transmission given that we know people are most contagious just before or at the time symptoms develop, so have we missed the boat? In fact, the authors of this analysis say in the UK, 44% of transmission occurred before symptoms developed.

As with the UK healthcare workers, infectivity is closely associated with two main factors, viral load and frequency of contact.

Reassuringly, the evidence to date on COVID-19 suggests that those people with fewer symptoms have a lower viral load.
“Recent data from COVID-19 have shown that those with severe infections had viral levels 60 times higher at presentation than those with mild disease,” the researchers wrote.

Unfortunately, at this stage we cannot easily determine viable infecting dose. And someone who presents with mild symptoms could develop more severe disease later down the track, but the general principle is worthwhile noting.

The other factor is, of course, frequency of contact and that’s both personal contact and environmental contact. This is where the analysis authors say we can look to good evidence to justify the behavioural interventions that most health authorities are advocating.

Back in the swine flu pandemic, the researchers were involved in the randomised trial of Germ Defence, a website providing advice on infection control measures. These measures included basic behaviours such as handwashing but also practical ways to reduce exposure in households where one member was affected such as cleaning of shared surfaces and adequate ventilation. It may all seem logical but this study of over 20,000 people proved that these measures reduced the risk of catching the respiratory infection significantly.

While the authors are quick to point out that we shouldn’t assume what works for one virus will work for another, they do say this study supports the belief that viral load is important in COVID-19. It also provides the evidence-base for the directive that ‘people caring for household members who are unwell should be encouraged to take measures to reduce the infecting viral load to reduce the incidence and severity of the disease.’ A principle that most GPs in Australia are adhering to in their current pandemic practices.

BMJ 2020;369:m1728 | doi: 10.1136/bmj.m17282

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