Let's call today day one. Suppose an individual, we'll call him Jack, becomes infected through contact with a person who caught the virus a few days earlier. Jack becomes infectious to others on day four, but without symptoms appearing on that day. The symptoms, such as coughing, difficulty breathing and fever, begin on day five and Jack remains infectious and in the community through day nine. At that point most individuals recover or at least become much less infectious to others. The unlucky ones end up in hospital and in the most severe cases, die. Such numbers will vary by case, but are broadly in line with the

During days four through nine, Jack circulates in the community, especially if his symptoms are on the mild side, potentially passing the infection to others. Let us say that he makes on average 16 daily contacts with others, some briefer, some longer. Most of those contacts don't infect the other person, but every once in a while the contact does infect the other person, at work, in the theater, across the dining room table, or in some other place.

Jack is infectious to others for six days (during days four to nine), during which he makes 96 contacts in total, and infects 2.4 other individuals (the reproductive number, or the number of secondary infections generated from one infected person, of the novel coronavirus -- is thought to be between two and 2.6, according to models from Imperial College ). In this scenario, that comes out to about 40 contacts for every one infection, again a rough number broadly consistent with the evidence about the rate of disease transmission.

There are two main ways to stop the pandemic. The first way -- the approach now widely used around the country -- is through an economic lockdown and "shelter in place." By deliberately closing down most businesses, the daily contacts per person fall. Suppose that falls by more than half, say to six per day. Over six days, Jack now only makes 36 contacts rather than 96, and as a result now infects on average less than one other person.

This is why the lockdown works, if it is enforced. Over time, the number of infected people declines as those currently infected recover, followed by the smaller number of people they have infected. The lockdown policy, however, comes at a high short-term cost. Daily life and the economy are put on hold. Incomes, jobs, tax revenues, and businesses could collapse during the lockdown.

There is a much better way -- when it is feasible. Suppose that on the evening of day five, after two days of symptoms, Jack is visited by a public health worker who has conducted vigorous contact tracing. The health worker explains, "We are here because your colleague at work developed a mild Covid-fever three days ago. Do you also have symptoms?"

"Well, yes," comes Jack's reply. "I woke up today feeling not so great, but thought I could shake it." The health worker is able to perform a diagnostic test and Jack remains isolated at home until the test confirms the Covid-19 infection. After a positive diagnosis, he is instructed to stay at home for the recommended stretch after his symptoms disappear.

Jack is informed that he will receive full sick pay as long as he sends a text message of his temperature every six hours and as long as he abides by self-isolation or, if he's not able to text, he will be visited daily by a community health worker to monitor his temperature and condition. He is also given an emergency call-in number and a website for food deliveries if necessary. The health worker leaves some face masks in case Jack must have any contacts with others, such as a family member or delivery service. The health worker also leaves a thermometer if one is needed.

If enough newly infected people are visited early on by public health workers, or go quickly into self-isolation before infecting other people, the pandemic will continue to slow and then stop.

Jack has been caught by the public health service after just two days of being infectious, and so will have made an average of only 32 contacts (16 contacts a day for two days), even less than the 36 contacts we've assumed he would make during 6 days of partial lockdown (six contacts a day for six days). This containment system can find a substantial proportion of symptomatic cases as long as the lockdown phase has done its job of repressing the pandemic.

And for those the system does not catch, individuals themselves can step up their own response. Those with symptoms can call a Covid-19 hotline to arrange for a rapid diagnostic test at home or a pharmacy. The outcome should be the same: early isolation so as not to infect others. Of course, individuals should have guaranteed paid sick leave and free testing so that they can afford to self-isolate.

The newly scaled up public health system, augmented by heightened self-monitoring by the public at large, can keep ahead of new infections. If enough newly infected people are visited early on by public health workers, or go quickly into self-isolation before infecting other people, the pandemic will continue to slow and then stop. Other steps, such as wearing face masks, monitoring temperatures in crowded public places, and placing hand sanitizers in many locations, could also help.

There are many uncertainties and details. How fast can the system track and isolate infected individuals? What limits on big gatherings should remain to prevent "super-spreading" of the virus? Yet the basic logic should be clear: the lockdown phase must be followed by a containment phase based on the public health system. There's also the important factor of asymptomatic individuals, who, could make up as much as 25% of all infected individuals, according to the director of the Centers for Disease Control and Prevention.

While some of these people may be caught by contact tracing and tested, others will not. To compensate for asymptomatic transmission, a high proportion of the symptomatic individuals must isolate early, either through contacts by the public health system or by their own initiative.

Europe and the US did not have effective public health systems in place at the start of the pandemic. In contrast, some countries in East Asia, such as Singapore Hong Kong, and Taiwan, have highly effective public health systems, with experience after the 2003 SARS outbreak. By taking a proactive approach to testing and tracking the infection, and by promoting personal hygiene (such as hand washing) and widespread temperature monitoring, they by and large succeeded in isolating a significant share of infections. As a result, they have so far kept the confirmed cases per 1 million population far below the levels of the US and Western Europe.