More on the “second wave”

The governments and the public have been remarkably successful in slowing down the spread and, in some cases, locally eradicating SARS-CoV-2 virus. The strategy of locking down the whole country – aimed at strongly reducing the potential of the virus to spread between infected and susceptible individuals – has worked in most places. Understandably, many countries and regions are now keen to relax the lockdown measures, to restart the social life and economics of the country. However, if the restart is not done “properly”, it can lead to another, perhaps even more dangerous outbreak.

Epidemiological concepts, such as the reproductive number, R, can be useful in understanding the dangers of the “second wave” and to signal when appropriate measures to stop it need to be implemented. I have written an article for The Conversation using simple models (detailed on this site) explaining how the “second wave” could look like.

The article has been attracting comments and questions and I will use this blog to answer some of them.

Is the “second wave” inevitable?

I do very much hope it can be avoided, but being realistic, I think we are going to see the virus returning over the next months, if not years. In an ideal world, we should be able to open shops, factories and care homes to such an extent that no significant increase in the number of cases occurs. Any new cases would be detected quickly and isolated, and all vulnerable people will be protected as needed. But real life does not work this way.

As I said in my article, there are still many susceptible individuals and very possibly still (too) many infectious ones, possibly non-symptomatic and hence difficult to detect. Even in countries or regions that seem to have eradicated the disease, there is always a possibility that somebody will bring it from abroad once international travel is allowed. I think that we are going to see a number of relatively small outbreaks, hopefully against a backdrop of a general decrease in the cases. Thus, the best we can do is to engage in a strategy of “whack-a-mole” and try to stop the (hopefully small) outbreaks from spreading further.

Quoting Neil Ferguson:

One of the key issues is how much will relaxation of current controls lead to potential increases in transmission. I should say that it is not clear that it automatically will do so. But most of the modelling would suggest that we might get a small increase, but it’s very unclear whether that will lead to the reproduction number being [above 1] ….

I suspect, though, under any scenario that levels of transmission, and numbers of cases, will remain relatively flat between now and September – short of very big policy changes, or behavioural changes in the community.

The real uncertainty is if there are larger policy changes in September, as we move into the time of year when respiratory viruses tend to transmit slightly better, what will happen then? And that remains very unclear.

Neil Ferguson, 2nd June 2020, after The Guardian

However, my main concern at the moment is twofold. Firstly, that the current changes to the lockdown policies will move us above the R=1 threshold and once we are there, the increase could be very fast and hence difficult to contain. Secondly, and more importantly, that once we are in such a situation (with R above 1), it will be very difficult to put the country under the severe lockdown again. In such a situation we will probably need to come up with novel ways of controlling the disease. Regional, and possibly more importantly, sectoral lockdowns will become much more important (like protecting care homes in a much better way). But also, that we will have to adapt to live with the virus for a long time (see my The Conversation articles on the long-term predictions and on the potential strategies to apply vaccine if/when it becomes available).

Can we predict the “second wave”?

To do this we would need to know very exactly how to link people’s behaviour to disease spread and to be able to account properly for all uncertainties associated with this. The model I used for the article (described elsewhere on this site) is extremely simple and deterministic and assumes that we can project the value of R into the future. The Agent Based, or Individual-Based, or (many) network models, in contrast, should be able to translate assumptions made about the societal response (e.g. in terms of mobility patterns) into the number of cases. Indeed, this is what Neil Ferguson’s model does, if I understand it correctly. But, even these more complex models are not a “crystal ball” and are dependent on primary data and on modelling assumptions.

Is the virus changing?

There has been reports that the virus is getting less virulent and hence the “second wave” is not going to happen. I am personally sceptical about such isolated reports. Although there is some evidence that pathogens might tend to change towards lower pathogenicity, the time scale seems a bit too short for this. Also, these reports come from a place which experienced very high infection pressure, unlike the UK, for example, so not sure how applicable this will be elsewhere. But to repeat what is said above, we are probably looking at a bit of a “whack-a-mole” “second wave” in the next few months, with hopefully a vaccine or an effective treatment coming online afterwards.