JUDITH GLYNN: When and why should we isolate people and how long should we isolate them for? This depends on three key features of infectious diseases– the incubation period, the latent period, and the infectious period. In this lecture, we’ll look at all three and how they interact.

In October 2014, American nurse Kaci Hickox was placed under quarantine for 21 days when she returned from West Africa to the United States. She protested and eventually got the court order overturned. There was a lot of media interest and strong feelings on both sides. So who was right? And why was 21 days chosen? To understand this, we need to look at the key time periods for an infection– the incubation period, the latent period, and the infectious period.

Starting with the incubation period. This is the time from when infection’s first acquired to when someone first shows clinical symptoms. It tells you something about the disease. The latent period is a time from when the infection’s acquired to the onset of infectiousness to the time when someone can first infect somebody else. And this is followed by the infectious period, the time during which an infected person can transmit to others. These tell you about the infection. So we have the incubation period, which tells you something about the disease. And the latent and infectious periods tell you about the infection. Let’s think a bit more about the incubation period. This is characteristic for each disease, but there is variation.

Each disease will have a characteristic minimum and maximum and average incubation period and the distribution of incubation periods is usually approximately log normal. That’s to say it has a single peak and a long right tail. And the incubation period may be influenced by the infecting dose. People who have a higher infecting dose tend to have a shorter incubation period. An incubation period is important for helping us work out when, and therefore where, an infection occurred. It also lets you know how long to quarantine someone after they’ve been exposed before you know whether they’re going to get ill or not. This shows the typical shape of an incubation period with a single peak and a long right tail.

This graph comes from an outbreak of salmonella in the UK, and what’s plotted is actually the onset times when people first got ill. Because everybody ate dinner at the same time, the distribution of onset times is actually the distribution of incubation periods. And in this outbreak, people who ate two pieces of the chicken, which is where the infection was, tended to have a shorter incubation period than those who’d only eaten one. So what about the latent period? This is the time from when the infection’s acquired to the onset of infectiousness. Time when people can first transmit the infection to others. And then this is followed by the infectious period, when transmission can go on occurring.

And people can have different levels of infectiousness during this period. And it determines how infection will be transmitted in the population and how long patients who are infectious should be quarantined for. So how do the incubation period, latent period, and infectious period fit together? Well, it can be in different ways depending on the disease. So disease of Type A shown at the top, the incubation period is longer than the latent period. What that means is people get symptoms only after they’ve become infectious. So people could start transmitting before they have any symptoms of disease, when they still appear to be totally well. That helps an infection to spread because people are out and about. They don’t know that they’re ill.

And it’s one of the reasons that measles spreads easily and is difficult to control. Diseases of Type B shown at the bottom, the incubation period is shorter than the latent period. That means people have symptoms before they’re infectious, before they can transmit to other people. SARS, the Severe Acute Respiratory Syndrome, is an example of disease of this type. So what about Ebola? This shows the incubation period of Ebola in the first outbreak in 1976. The shortest incubation period was two days, the longest 21 days, and there was a distribution in between. And this same pattern has been seen in subsequent outbreaks, with an average of around eight to nine days.

In Ebola, the incubation period is shorter than the latent period. That means that the symptoms start before someone becomes infectious, before someone could transmit to somebody else. So people who are asymptomatic can not transmit. During the infectious period, the infectiousness increases as the disease gets more severe. And actually, it continues after death, as dead bodies are still infectious. For survivors, there’s still some infectiousness when they feel well for a short period. And infectiousness in semen continues for up to about three months.