As well as limits on gathering size in recent months ( https://twitter.com/AdamJKucharski/status/1298970718105993217?s=20 ), another feature of Sweden that stands out is household size - it's smallest average in Europe, with majority single occupant ( https://ec.europa.eu/eurostat/web/products-eurostat-news/-/DDN-20170905-1). What effect could this have on transmission? 1/

The estimated risk of transmission per contact is higher within household than outside household ( https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30457-6/fulltext). This means it can be helpful to think of an epidemic as a series of within-household outbreaks, linked by between household transmission... 2/

If the average size of an outbreak in a household is H, and each infected person within a household spreads infection to C other households in community on average, then we can think of the 'household' reproduction number as equivalent to H x C. 3/

So if household size is smaller in a particular location, we'd expect H to be smaller, and hence lead to a lower reproduction number, even if C is the same... 4/

Now, of course, there are many, many other factors that influence transmission (behaviour, control measures, immunity, perhaps other seasonal effects). And it's important to compare countries & understand what might be happening – and what control measures could be adapted... 5/

But before claiming 'what country X is doing would have same effect here', we need to consider whether there are key differences in population stucture between countries that might change effectiveness if same measures used elsewhere. 6/6