London is in a precarious position when it comes to Covid-19. Remaining restrictions were recently lifted in the city along with the rest of Britain even as the highly infectious Delta variant drives cases higher.

London's Delta wave will not be as bad as past outbreaks — hospital admissions and deaths remain low because of its high vaccination rate. But the trend is moving in the wrong direction. New cases are averaging about 5,000 a day, people are isolating at high rates, and hospital admissions are beginning to tick up. The reaction to loosened restrictions has been muted and often critical.

A big question is whether New York, another large city that has reopened after having been through the Covid wringer in 2020, will face a similar resurgence of cases that could thwart its return to normal. The answer is likely yes — and there’s an even chance it could be worse than London’s in some ways.

New York is much better positioned than many other parts of the country and the world because of its relatively high vaccine uptake. But the Delta variant is especially adept at finding gaps in protection. And relative to its counterpart across the pond, New York has some cracks in its shield.

The first weak spot is New York’s incomplete protection of people older than 65, who are particularly vulnerable to Covid. As of Thursday, 72 per cent of that population was fully vaccinated. In London, the number is 85 per cent, which is likely one of the reasons it has managed to keep hospital admissions low so far. New York’s success in this older population is also uneven. The rate of full vaccination is just 64 per cent in Brooklyn, for example; only one of London’s 32 boroughs, Westminster, has a rate that low, while ten are above 90 per cent. In an outbreak of similar size, New York may see more bad outcomes.

The second potential vulnerability is New York’s slowing vaccine momentum, which represents a lost opportunity to tamp down Delta’s spread heading into the riskier autumn season. While New York has a higher vaccination rate and is in a better position than London was when its cases began to tick up, many more Londoners have received first doses and will soon get second ones. That will push the city’s numbers past New York’s soon, even though Britain is not yet broadly vaccinating children, and will give it a better chance to shorten its outbreak. The Bronx, where only 45 per cent of people are fully vaccinated and 49 per cent have at least one dose, is falling farthest behind.

London is doling out more than twice as many daily doses as New York City on average. It is exactly the wrong time for shots to stall. Delta's transmissibility means that even the oft-cited 70 per cent threshold for herd immunity is likely too low and that unvaccinated people are at especially high risk.

None of this is to suggest that New York needs to panic or lock down. Vaccines still prevent Delta from causing severe disease, and most vaccinated New Yorkers have received highly effective mRNA shots. But the present aura of safety is fragile, and Covid is not finished with the city.

It should be prepared to put targeted restrictions in place when warranted and use the limited window before the variant firmly establishes itself to redouble efforts to reach the people it has missed. Fighting Covid is a marathon, not a sprint.

• Max Nisen is a Bloomberg Opinion columnist covering biotech, pharma and healthcare