Still, it wasn’t easy to get people talking about their sex lives. Interviewers had to persuade people to take part – often by emphasising the benefits for wider society – and build enough trust for participants to answer honestly. Then there was the issue of sexual terminology. ‘There was that mismatch between the public health language and the language of everyday, which was so full of euphemisms,’ Wellings noted. She recalled that several participants didn’t recognise terms like ‘heterosexual’ or ‘vaginal’. ‘All the Latin-sounding names, or any word with more than three syllables, was thought of as something completely weird and unorthodox.’
Yet the Natsal team did have some advantages, such as the relatively low frequency of sexual encounters. The most recent Natsal study found that a typical twenty-something in the UK has sex about five times a month on average, with less than one new sexual partner per year.[12] Even the most active individuals are unlikely to sleep with more than a few dozen people in a given year. It means that most interviewees will know how many partners they’ve had and what those partnerships involved. Contrast that with the sort of interactions that might spread flu, such as conversations or handshakes. Each day, we may have dozens of face-to-face encounters like these.
During the past decade or so, researchers have increasingly tried to measure social contacts that are relevant for respiratory infections like flu. The best known is the polymod study, which asked over 7,000 participants in eight European countries who they interacted with. This included physical contacts, like handshakes, as well as conversations. Researchers have since run similar studies in countries ranging from Kenya to Hong Kong. The studies are also getting more ambitious: I recently worked with collaborators at the University of Cambridge to run a public science project collecting social behaviour data from over 50,000 volunteers in the UK.[13]
Thanks to these studies, we now know that certain aspects of behaviour are fairly consistent around the world. People tend to mix with people of a similar age, with children having by far the most contacts.[14] Interactions in schools and at home typically involve physical contact, and encounters that occur on a daily basis often last longer than an hour. Even so, the overall number of interactions can vary a lot between locations. Hong Kong residents typically have physical contact with around five other people each day; the UK is similar, but in Italy, the average is ten.[15]
It’s one thing to measure such behaviour, but can this new information help predict the shape of epidemics? At the start of this book, we saw that during the 2009 influenza pandemic, there were two outbreak peaks in the United Kingdom: one in the spring and one in the autumn. To understand what caused this pattern, we simply need to look at schools. These bring children together in an intensely social environment, creating a potential mixing pot of infection; during the school holidays, children have around 40 per cent fewer daily social contacts on average. As you can see from the graph above, the gap between the two pandemic peaks in 2009 coincided with the school holiday. This lengthy drop in social contacts was large enough to explain the summer lull in the pandemic. However, school holidays can’t fully explain the second wave of infection. Although the first peak was probably due to changes in social behaviour, the second peak was mostly down to herd immunity.[16] The rise and fall of infections during school terms and holidays can influence other health conditions too. In many countries, asthma cases peak at the start of a school term. These outbreaks can also have a knock-on effect in the wider community, exacerbating asthma in adults.[17]
Dynamics of the 2009 influenza pandemic in the UK
If we want to predict a person’s risk of infection, it’s not enough to measure how many contacts they have. We also need to think about their contacts’ contacts, and their contacts’ contacts. A person with seemingly few interactions might be just a couple of steps away from a high transmission environment like a school. A few years ago, my colleagues and I looked at social contacts and infections during the 2009 flu pandemic in Hong Kong.[18] We found that it was the high number of social contacts among children that drove the pandemic, with a drop in contacts and infection after childhood. But there was a subsequent increase in risk when people reached parenthood age. As any teacher or parent will know, interactions with children means an increased risk of infection. In the US, people without children in their house typically spend a few weeks of the year infected with viruses; people with one child have an infection for about a third of the year; and those with two children will on average carry viruses more often than not.[19]
As well as driving transmission in communities, social interactions can also transport infections to other locations. In the early stages of the 2009 flu pandemic, the virus didn’t spread according to the as-the-crow-flies distance between countries. When the outbreak started in Mexico in March, it quickly reached faraway places like China, but took longer to appear in nearby countries such as Barbados. The reason? If we define ‘near’ and ‘far’ in terms of locations on a map, we’re using the wrong notion of distance. Infections are spread by people, and there are more major flight routes linking Mexico and China – such as those via London – than those connecting Mexico with places like Barbados. China might be far away for a crow, but it’s relatively close for a human. It turns out that the spread of flu in 2009 is much easier to
