Tuesday, 19 May 2020

Is COVID19 seasonal? Will it just go away?

The most optimistic view of the pandemic's epidemiology is that it will simply peter out in the summer and disappear after effective lockdown measures have done their job. 

Alternatively, there are those who state that the lockdown has made no difference at all and that nature is following its course.

So I decided to look at whats been happening and find out, as best as I can, where the truth lies. First, some good news from the front line:

Good news from London


A recent study of health care workers in a London hospital shows that the decline there is steeper than elsewhere in the country. The the red line shows the number of COVID positive patients and the blue, the COVID positive staff (right axis) in that particular hospital.  The brown bars shows the total number of positive patients in greater London (Left axis).

So, rates of infection are falling in the community, the staff and in patients.

This is good news, optimists predict that at his rate there may soon be no new cases in London, though I suspect there will continue to be outbreaks. 


Epidemioloigy in London

Can this all be due to the success of the lockdown? I doubt it's the whole story.


So what is the effect of seasonality?


The seasonality of flu is self evident but that doesn't mean a different virus will behave the same way. Might a recent novel coronavirus outbreak in Germany give us a clue?

This graph show what happens to just such a novel coronavirus, a less disagreeable cousin of COVD19 which caused croup in Germany. It was called HCoV-NL63, and this outbreak took place during the winter of 2000. It clearly declined over March and disappeared by April.

COVID19 is far more transmissible and arrived here later in the winter, but you could argue that it might be behaving in similar way; fading out as the spring springs, the sun comes out and as vitamin D levels increase.



What coronaviruses tend to do


If COVID19 is behaving in anyway like this, then we could expect a natural decline at this time of the year. If true, this is also good news, but watch out for next winter. 


Comparing countries


Rates of infection are falling across all countries in the northern hemisphere. In some ways this might be what you would expect for a highly transmissible seasonal viral infection even without lockdown. The graph below shows the number of daily cases per million in the 10 hardest hit countries so far. 

I can't help but be struck by the similarities of the overall shape between countries, despite their differing approaches. Rapid increases, peaks in April and then variable declines. Interesting! 

 The dreaded "league table" is shown on the right side. 




In terms of daily deaths per million some differences do emerge, and the differences in outcome of different approaches become more clear, again the 'league table' is on the right. 


It seems to be that there are two things going on, similarities due to the natural history of the disease and different rates of control due to policy decisions.

If there is an effect of seasonality, the next place to look is the other side of the globe, as they enter their winter season.


What about Southern hemisphere?


The patterns here are worryingly different. The bizarre behaviour of Bolsonaro at the helm in Brazil is accelerating deaths there, but the rates do generally seem to be climbing as they approach their winter, so seasonality is important. The impacts will be different too, with many more young people, but far less capable pubic services.

Also evident is that early aggressive measures in South Africa, New Zealand and Australia seem to be keeping their outbreaks under control. Again, a mixed picture.




Global cases are sadly, still increasing. 

Seasonality, Social distancing and Lockdown


So as ever, the middle way seems to hold true; there is a definite effect of seasonality on the transmission of COVID19, social distancing and lockdowns have had effects too. For politicians to claim they have in any way 'beaten' COVID19 is nonsense, as is the claim the lockdown has made no difference. It's true to say that:

  • The more humid warmer weather viruses generally dislike is associated with a natural seasonal decline. 
  • More sunshine and consequent increasing levels of Vitamin D are important. Some think, with much evidence, this is the main factor, I covered this in a previous post and update and I will re-visit this again as the summer goes on and more evidence comes in.  
  • Social distancing and lockdown reduce transmissibility too, the earlier the better. 

At the same time, life is creeping back to the 'new normal' with increasing numbers back at work and more leisure. We are entering new territory, yet the decline in cases in the UK is painfully slow with implications for the future.  So what is going to happen now?

COVID and the summer.



The graph show the latest prediction of when the wave will finally end. I have been watching these particular number crunchers; the date they predict the end of the wave is moving further into the autumn as our cases decline far more less steeply than they increased at the start of the pandemic. 

The graph is already a little behind, but there are still 3000 test-positive cases daily in the UK. A prominent member of the iSAGE group, John Edmunds suggests the number is closer to 20,000 a day. The end is now predicted to be in September.


End of the first wave

COVID19 and next winter


If this is true, it means COVID19 will be around when:
  • The air becomes dryer and colder
  • Children head back to school
  • Students head off to university 
  • Vitamin D levels start to fall

So truly comprehensive systems to control local outbreaks will need to be ready. Early and perhaps more sensitive focused restrictions might be needed, perhaps on an local or regional basis. Testing and tracing will have to be rapid and efficient and we will have to plant some more money trees to fund it all.  Thats another story I will cover in another post.

Meanwhile in a distant pig factory in Utah, or a poultry factory in Guangdong, viruses are meeting and mutating into new and novel forms. Thats another story too.......

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