The three phases of Covid-19 – and how we can make it manageable
How is the coronavirus likely to play out, how does it end, and does our behaviour make a difference? Here infectious diseases expert Dr Siouxsie Wiles walks us through the epidemic curve, with illustrations by Toby Morris.
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In a post last week, I showed you a picture of how the Covid-19 outbreak has played out in China. It was a graph of the number of cases reported each day, something we call an epidemic curve. It looked a bit like this. We can use the epidemic curve as a visual way to think about how Covid-19 could go, both here in Aotearoa New Zealand and globally.
But before we get to that, let me just explain epidemic curves in a little more detail. First, they have three parts: a start (phase one), a middle (phase two), and an end (phase three). Each of these phases is influenced by different things. Such as, how infectious is the virus or bacterium? How is it transmitted? Is it person-to-person, or from eating or drinking? If it’s person-to-person, are people infectious before they have symptoms and don’t know they are unwell? And if people do have symptoms, are they mild enough for people to go about their daily routine? Is everyone susceptible to the infection, or just some people? What resources do we need and have to contain the outbreak? The answers to these questions will influence the shape of the epidemic curve.
Phase one of the Covid-19 epidemic curve – containment
At the moment, Aotearoa and many other countries are in phase one for Covid-19. This is the start of the curve where cases pop up sporadically as people who have contracted the virus in one country travel to other countries.
Think of Covid-19 as several fires blazing away, with embers shooting off in all directions. Our goal is to stop those embers from turning into another blazing fire. It’s beginning to look like anyone who has travelled overseas recently should think of themselves as a potential ember and be on the lookout for symptoms.
Sticking with the fire analogy, it’s not unusual for embers to smoulder a little when they land. That’s why public health officials are actively looking for people who have been in close contact with someone with Covid-19. This is called contact tracing. Anyone at high risk of having contracted the virus is then put into isolation. Here in New Zealand, two of the five people who have tested positive for the virus so far are partners or relatives of those “embers”.
Covid-19 is now officially a pandemic. Do not panic. But do draft up a pandemic preparedness plan. Here’s Siouxsie’s step by step guide.
Our aim is to stay in phase one. The longer we can stay in phase one, the better we can prepare for phase two. If we can stay here for the next one to two years, then hopefully a vaccine will become available and we can avoid phases two and three altogether. One way to do that would be just to ban all international travel. No one in or out. Just to be clear, I don’t think this is the right approach to take. One to two years is a long time.
Alternatively, we could just make everyone who travels here, Kiwis included, go into isolation for two weeks. Again, I’m not sure how well this would work in practice. My advice to everyone, even if you haven’t travelled, is to be on the lookout for any of the symptoms of Covid-19. If you have a fever, or a cough, or shortness of breath, stay away from other people if you can. If you’ve a runny nose or sore throat this could be a really mild form of Covid-19 so I would also isolate yourself.
Phase two – community transmission
The way Covid-19 is playing out globally, we are going to enter phase two at some point. This will happen when people who don’t realise they have contracted Covid-19 go about their daily lives rather than stay isolated. More and more cases will be reported each day as the virus transmits from person to person out in the community and outpaces our ability to keep up with the contact tracing. The quicker and higher the numbers rise, the more likely the outbreak will overwhelm us, making it harder to control.
The data so far suggests all of us are susceptible to the virus. It’s clear from what’s happened overseas that many people with Covid-19 spent many weeks being treated in hospital before they recovered. This isn’t a trivial disease. China built two new hospitals in a matter of weeks to keep up with demand. Do we have the resources to do that if it came to it?
Flattening the curve
Our aim will be to keep phase two of the epidemic curve as flat as possible, keeping the number of cases reported each day as low as we can. If we can achieve that, it’ll mean we’ll be able to treat everyone who needs treating. We can all help with this by washing our hands regularly, avoiding touching our mouth, nose, and eyes, and staying away from other people when we are sick. This also means calling ahead if you feel sick and want to go to the doctor or hospital. The last thing we need is loads of our healthcare workers in isolation because they’ve been exposed to Covid-19. Forty-five staff from North Shore Hospital have been isolated as a precaution because of a probable case.
Another thing we are all going to need to start doing soon is minimising or avoiding contact with other people. This is called social distancing. If you are greeting people, don’t hug, shake hands, hongi, or kiss. Bump elbows or feet instead. Work from home if you can. Much as it pains me to say it, social distancing also means avoiding public transport (get on your bicycle!). Similarly, it means avoiding gyms, churches, cinemas, concerts, and other events and places where people congregate. At the community level we may need to close schools, universities, museums, and workplaces, limit public transportation, and cancel public events. This is what China did so effectively and what Italy is currently implementing in some regions.
After ‘Flatten the Curve’, we must now ‘Stop the Spread’. Here’s what that means
Moving into phase three
We move into phase three when the outbreak is either brought under control or everyone has been infected and there are no more susceptible people left to infect. Vaccination is one way we can stop people being susceptible, or at least enough people that the disease stops spreading from person-to-person – that’s what herd immunity is all about.
Given we are one to two years away from a vaccine, bringing Covid-19 under control is obviously the goal here and what China has achieved. But if there are still susceptible people in a population and we stop taking all the right measures then we will see cases flare up again.
That’s why what happens on the global scale is so important. Any “fires” left blazing threaten us all. At the moment, many of the new cases being reported in China are people who have contracted the virus while travelling outside of China. They are essentially back in phase one but with all the understanding of why it’s so important to stay there. And all the expertise and public knowledge to make it happen.
While the coronavirus responsible for Covid-19 likely started out in bats, so far, it’s not clear how it got from bats to people. SARS jumped from bats to humans via civet cats. With SARS once we managed to stop all human-to-human transmission, the disease then ran its course in those infected, and we haven’t seen SARS since.
Hopefully, if we can stop all human-to-human transmission of Covid-19 that’ll be the end of this coronavirus too. But it’s clear that we’ll all have to play our part to make that a reality.