London and New York have similar populations and density, but only one is seeing a major coronavirus surge. Here's why.

• New York City and London are similar in density and population, but their coronavirus outbreaks have progressed very differently since the spring.
• Over the summer, New York developed its own robust testing and contact tracing programs and implemented targeted lockdowns in neighborhoods with new outbreaks.
• London encouraged dining at restaurants and relied on a national contact-tracing program.
• New York is now seeing a mild uptick compared to its spring catastrophe, whereas London is counting more cases than ever.

When the coronavirus first reached the US and England, the countries' largest cities reacted similarly: New York City issued a stay-at-home order on March 22, and London followed suit March 26.

But since then, the two cities' pandemic trajectories have diverged.

New York City was the US's first hotspot, peaking at over 6,000 daily cases in April (though many were missed due to limited testing) and more than 500 deaths per day. At that time, London kept new cases to about 900 per day.

Throughout the summer, both cities kept the virus at a simmer: London averaged dozens of new cases per day and New York a few hundred. But starting in August, London's numbers crept – then catapulted – beyond the scope of its first outbreak. By November 5, the city averaged nearly 2,000 new cases per day.

New York's numbers, meanwhile, have stayed relatively low. By November 5, the city had 745 new cases per day, on average.

Experts say London's recent spike came because officials sent residents mixed messages, didn't implement enough contact tracing, and quickly reopened restaurants, bars, and gyms. By contrast, once New York got its spring surge under control, it implemented widespread testing, contact tracing, and targeted lockdowns. Leaders also developed consistent messaging about necessary precautions.

New York was an anomaly in that, according to David Heymann, an infectious-disease specialist at the London School of Hygiene and Tropical Medicine.

"Everybody locked down without exit strategies. And when they opened up, they opened up overnight," he told Business Insider.

London reopened quickly and didn't enforce mask-wearing

New York and London are both global hubs with 8 to 9 million residents, diverse demographics, and dense apartment buildings. But whereas New York set the terms of its own lockdown, London's response has depended mostly on guidelines set for the entire UK.

London reopened several key economic sectors in step with the rest of England, even the country's sparsely populated areas. Indoor gyms throughout England reopened on July 25 – a month before New York's did. The country also allowed pubs and restaurants everywhere, including London, to open for indoor dining and drinking on July 4. New York waited nearly three more months — until September 30 — to allow indoor dining and still hasn't reopened bars.

The UK government even encouraged residents of England, Scotland, and Wales to frequent restaurants.

In July, Rishi Sunak, head of the UK's treasury department, announced "Eat Out to Help Out," a plan that encouraged residents to dine at restaurants by offering them 50% discounts on Mondays, Tuesdays, and Wednesdays in August. Restaurant occupancy skyrocketed: By the end of that month, more people were eating at restaurants on Mondays, Tuesday, and Wednesdays than on those same days in August 2019.

It's hard to determine precisely how much "Eat Out to Help Out" facilitated the coronavirus' spread, but one researcher estimated the initiative could have caused almost a fifth of new clusters over the summer.

London's hospitalization and death rates haven't skyrocketed in this second surge the way its case counts did. The city is seeing an average of just under 17 deaths per day, which is low though higher than New York City's average of about 8 deaths per day. The reason for this discrepancy is that the bulk of London's new cases are being recorded in young people, Heymann said. According to the UK's COVID-tracking website, people between 20 and 39 have contracted most of the city's cases overall.

Heymann added, though, that it's possible the virus could trickle through to older populations in the coming weeks.

Targeted lockdowns prevented major surges in New York City

In early October, New York officials instituted two-week lockdowns in specific neighborhoods of Brooklyn and Queens where COVID-19 cases were spiking. They closed schools, indoor dining, and non-essential businesses according to the severity of each area's numbers.

As a result, new outbreaks haven't spread beyond those boroughs, according to Maureen Miller, an epidemiologist at Columbia University's Mailman School of Public Health.

"You don't have to close down the whole city," she told Business Insider. "I think that's a very different approach than other US cities have taken and certainly than London has taken."

Miller said the UK government, by contrast, has operated with an "on/off switch," either reopening almost completely or instituting widespread lockdowns. That approach can fatigue residents in areas with little spread while failing to sufficiently curb new outbreaks.

British officials did weigh a targeted strategy like New York's: Prime Minister Boris Johnson said he wanted a "local, regional approach" on October 14, according to the Associated Press.

"They were looking at precision and were going to do it. But for some reason, a political decision was made to lock down," Heymann said.

England announced a nationwide lockdown on October 31.

London has also struggled to increase its test availability. As late as September, locals complained of days-long waits to schedule a COVID-19 test, according to the AP. New York City, by contrast, rolled out a universal testing initiative in June, providing free tests to all residents at 150 sites. Now the city administers between 38,000 and 65,000 tests daily.

This robust testing means "you can do contact tracing and you can do targeted lockdowns," Miller said.

England's contact-tracing program still struggles to reach people

At first, New York struggled to get residents to respond to reach-outs from its 3,000-person contact-tracing team. But by September, the system was reaching 91% of all new COVID-19 cases, according to Modern Healthcare.

England's national contact-tracing system, NHS Test and Trace, hasn't seen that kind of improvement. In the program's first week in May, workers contacted 73.4% of new COVID-19 cases. By September, the rate had only risen to 74%.

British Communities secretary Robert Jenrick said in October that the government would like to give more control of contact tracing to local governments, whose systems so far have had a 97% success rate in reaching new cases, according to the Local Government Association. But that change hasn't happened yet.

New York provided clear, regular messaging

In the spring, New York Gov. Andrew Cuomo became famous for his daily coronavirus briefings, in which he presented case counts, death tolls, projections, and expert advice.

"People wanted to hear it. I mean, Andrew Cuomo became a star because of this," Miller said.

WiFi kiosks around the city also broadcast messages about wearing masks, washing hands, and staying home.

"I am so happy that New York City is an outlier in such a positive direction," Miller said. "I think that has a lot to do with the constant and clear communication by both the governor and the mayor, and the caution that has been taken in reopening the city."

No leader in London, by contrast, communicated with Cuomo's regularity or reach. Like many US residents, Londoners have expressed frustration at what they see as a lack of consistent messaging on the national level.

"Go to the pub, but don't come into contact with other people. Only meet in groups of six, but also sit in a restaurant with 30 other diners. Go to your office, but don't go by public transport. Listen to the scientists, except when we're ignoring them," Guardian columnist Imogen West-Knights wrote of the government's "mixed signals."

London mayor Sadiq Khan, too, has expressed frustration with the government's messages.

"There are too many examples of mixed messaging and confused communications, which is leading to the virus going up," he said in August, according to My London.