Covid-19: what do we know?

Submitted by martin on 1 November, 2020 - 10:12
The virus

George Davey Smith, professor of Clinical Epidemiology at Bristol University, spoke to Martin Thomas from Solidarity. Click here to view the video interview online: this is a tidied-up transcript.

What do we know about how and where the virus is transmitted?

All of this sort of knowledge is provisional and it's not incredibly well defined. But there are some things that are pretty clear.

Indoor transmission on average is higher than outdoors. It is known that the virus survives longer at low temperatures, and also at very low humidity or very high humidity. Cold, dry places or cold, very wet places can be bad. Several outbreak events have occurred in meat packing places across the world, for example

There's still considerable discussion on the relative contribution of large particles (from coughing and so on) and aerosol transmission. Aerosol transmission can occur, but the relative contribution is still uncertain.

But the greater transmission indoors and from being closer to people seem to be the main factors.

What about transmission via surfaces? And unevenness of transmission: the estimate that 80% of infections come from 10 percent of transmitters?

Transmission via surfaces would relate to the temperature and humidity. I think the general view would be that surface transmission won't be playing a major role, except in cold environments where the virus can survive a long time.

Most of the so-called super-spreader events, as far as I understand it, are not thought to be via surfaces, but more from situations where aerosol transmission is encouraged, particularly in enclosed and crowded places

The estimate is that 10-20 percent of human-human contacts account for 80 percent of transmissions. The first factor here is the extent to which a particular person is closely interacting with other people and is the source of infection for those people. Such contacts certainly make a contribution.The more contacts people have, the more likely they are to infect and get infected, even if everyone is transmitting exactly the same.

There is also a contribution of susceptibility to becoming infected being different between individuals, something that has been studied for a long time in, for example, mice colonies in which the risk of particular animals in a standardised environment becoming infected varies greatly.

What do we know about which social measures reduce transmission or slow transmission?

Anything that leads to fewer people being in especially close proximity will reduce transmission.

By now, people are wearing masks, and taking care in shops and on public transport is accepted, although there aren't going to be randomised controlled trials of those sorts of interventions.

Mask wearing might very well be more about protecting other people than yourself. You wearing a mask isn't necessarily getting you that much protection, but might be protecting other people from you.

Many estimates have to be made on the balance of probable beneficial effects and potential adverse consequences of interventions.

One of the first things that many governments, including governments which did well, like South Korea did first, was close schools in the belief that they were major transmission hubs. Was that right?

My opinion is it was a mistake to close schools, or certainly to close primary schools, because of the very clear evidence of the long term detrimental effects of that, which are worse for kids in worse social circumstances, with parents who have to do more than one job each to keep afloat, and so on.

The actual evidence for closing schools was not particularly strong. There was a systematic review in The Lancet early in the current pandemic, which reviewed all the prior evidence on that, but most of that was based around flu, and this isn't flu. It suggested that the evidence wasn't strong with respect to flu at least

What has become apparent with this coronavirus - and it was surprising to me - is that transmission wasn't multiplied through primary school age kids, whereas that happens on a large scale for example with common cold coronaviruses. The group with the lowest infection rate is primary-aged kids. Teachers don't appear to have an increased risk of infection, unlike other public-facing workers.

Long-term school closures, or even not that long-term, are detrimental to the futures of the kids, and in particular those kids in the least favourable social circumstances.

In terms of the education that was provided during lockdown, it was massively different between some public schools providing seven hours a day of Zoom tuition and state schools providing an email every so often. In terms of increasing inequality, I think school closures are particularly detrimental.

Some countries didn't close schools and did ok, and in some other countries which did well, like Norway, the people responsible for the science behind the policy have said that school closures were a mistake.

What other factors might shape the very different experiences with the virus of different countries, even in Europe? In addition to better social measures here and worse there, such factors as the level of the previous flu season have been cited.

There was an intriguing analysis which examined this. The idea that is that if you have a really bad previous flu season, then more of the vulnerable who might die from covid will already have died from flu. There are quite lot of deaths every year from flu, and in fact more deaths than get registered as being from flu. An elderly, frail person may die for example of something recorded as heart disease, though that was triggered by flu.

However I don't think the evidence for this prior flu hypothesis is strong. Probably one of the most surprising things to me from what's happened over the last nine months is that we've not seen what's called mortality displacement - that once the sick elderly die of covid, then later on, with those people being removed from the population, mortality rates go down sharply among the survivors in that age group.

You do see that in situations like heat waves in particular cities - a high spike in mortality, then a rebound reduction.

That's just not been seen with this virus to any extent in the UK, and it's surprising because of the age distribution of deaths. Just how steeply mortality increases with age remains under-appreciated, I think.

The modal age of death from Covid is well over 90 for women. Even now, not a huge proportion of the population survive to 90. Even though over-90s are a comparatively small group, the absolute number of deaths peaks at an age above 90 among women.

One statistic which differs markedly across countries is the median age of death from Covid compared to life expectancy. In the UK, the median age of death is about 85 and life expectancy is about 81. In the States, by contrast, the median age of death from Covid is is early seventies. Even when the the US had a lower number of deaths per person than the UK, those deaths were occurring at a younger age on average.

What do we know about what's needed to make a good testing, tracing, isolating operation? How well would it work if it were good?

You get the international comparisons, and people say New Zealand has done very well with test, trace, isolate, as has Hong Kong. Test, trace and isolate can be effective when you have low numbers of cases, and if you can close borders.

The UK centralised system has been a disastrous waste of huge amounts of money. With a local system, you have local knowledge. If you see cases in a particular part of town, the people have some idea of what the environment is like, of what family structures are likely to predominate, and to have some notion about how best to do that tracing in that context. You need local knowledge.Some form of centralisation of information is important for monitoring. But the practical tracing needs to be locally organised.

But anyway the number of cases we've now grown to changes things. With the level of cases we have now, test, trace, and isolate not a plausible way of driving down infection rates.

Definitely there was the possibility over summer, when we went down to pretty low rates. Not zero, but the rates just bubbled along, probably meaning that reintroduction of infection into different local areas was managing to keep the virus going.

In such a situation you can try go down to virtual local elimination. Once the reproduction rate was below one, if the efforts followed up long enough, then infections should disappear, without some form of reintroduction or a few people remaining infectious for long periods of time, etc. But in a country of 60 million such local reintroduction will obviously happen more than in one of four million like New Zealand. Germany was held up as having had a model response, but this has clearly deteriorated of late. The notion that the UK could be like some sort of South Korea miraculously air-dropped into the middle of Europe is a fantasy.

With the number of cases we've got now, test-trace-isolate becomes a performative act, not making a useful contribution at this stage. With the just announced new lockdown this time should be taken to stop the current meaningless activity and reconstitute a functioning system.

There may be a potentially useful contribution from "backwards" tracing, which relates to the discussion about heterogeneity of transmission and potential super-spreaders. If you get sick, it's sensible to find out who you got the infection from, because the person you got it off is likely to be somebody spreading a lot. And I think that "backwards" tracing is being implemented in places which are maintaining low infection rates.

A thing to remember about New Zealand is that the borders are closed. You can only get into the country with a two-week quarantine. They've pretty much closed the borders in Hong Kong, except for a just-opened border with Singapore, and ongoing plans for both to relax border restrictions with mainland China. Because of the dependencies of the Hong Kong economy, as a hub of many activities, long term border closures are difficult to sustain.

Then the question arises of whether one thinks it's a desirable way forward to have closed borders for years on end.

There will be some forms of vaccine rolled out. Many of the ones in development aren't sterilising, which means they don't actually stop people carrying infection and maybe transmitting it, they just stop them getting ill. Those vaccines don't push towards elimination. And a large proportion of people will not take the vaccine. The UK has almost as high a figure as in the US of people who say they won't be vaccinated.

The virus is just not going to be driven away by vaccination in a few months. The notions of keeping it maximally suppressed for a few months, and then we'll be fine, are naive.

What do we know about the which sections of the population are more vulnerable?

The biggest factor by far, massively above anything else, is age. It is difficult to find any condition except dementia which a greater age relationship.

And that's why the care homes situation in the UK was such a disaster. And in Sweden, by the way - that was why Sweden did badly compared to its neighbours. As you know, in the UK, elderly people were discharged from hospital, still infected, back to care homes. That was a disaster.

On top of age, there's occupation. If you're in an occupation which involves a lot of contact with people, then even with people wearing face masks, you're never going be able to get the risk down to zero. For public transport drivers, train conductors or whatever.

The risk is greatest in the jobs where the people have the least ability to distance, taxi drivers for example, though teachers do not seem to have a high rate of infection.

For people like me who can happily do their job from home, the risk is much lower. But if you work on zero hours contracts to feed your kids, and need to get out of your home, then that obviously increases vulnerability.

Also, men do worse than women across the age spectrum, and particularly when you get to older ages. Different ethnicities have differential risk, and that effect partly appears to be due to occupation.

What do we know about possibly effective measures for shielding the more vulnerable?

Shielding is really difficult. In an institution where everyone needs to be shielded, like an old people's home, then at least you know what you've got to do. I think it's not an unrealistic hope to get better shielding through rapid tests of infection, where you can get the results in minutes. And then, for example, people could be tested before visiting the at-risk.

One of the upsetting things in recent months has been hearing of the very elderly, often people with cognitive decline, not understanding why no one's coming to see them. Visiting was forbidden. Hopefully at some point there will come an effective way of testing people who come to visit, and the people who work in those at-risk locations as well.

And important too is employment protection. You can't expect people who need to work to get by to self-isolate, or to get tested if then they're just made redundant.

Shielding in the home is also obviously highly dependent on the conditions in which it's been carried out in. One possibility is that if rapid testing is available then with an elderly sick person, people can be tested before seeing them. Some countries have provided hotel-style accommodation for people in situations requiring isolation.

What do we know about treatments for those suffering from the virus?

Treatments have definitely improved, in particular with steroids and with the general management of patients. With the process of learning about a new disease, survival is unequivocally increasing.

With treatments, you absolutely need to have randomised controlled trials. Some have argued treatments should be used without trials on the basis that otherwise you're potentially keeping something valuable from people.

That's not defensible because some treatments might have detrimental effects. In the USA, I believe, well over a hundred thousand patients with covid have been treated with convalescent plasma without any trials, and now since the trials have been done, it doesn't look positive And then there's been the hydroxychloroquine fiasco.

There are now treatments which randomised controlled trials have shown to have a reasonably large effect. We would not have known that without the trials. If at the beginning we had asked the people who know about these things to bet on what was going to turn out to be successful, it probably wouldn't have been dexamethasone at the top of their list. This shows the importance of proper studies to document treatments.

The UK, with a national health service that nearly everyone uses, should be in the pole position to do those studies, because you should have a national level information system. One positive that could come out of a very negative situation is if such a system developed in response to the pandemic.

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