Researchers are trying to discern whether vaccines work on the Delta variant.,

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Credit…The New York Times

The C.D.C. estimates that the Delta variant is now the dominant one in the U.S.

More incarcerated people in the U.S. may have died from Covid-19 than previously known.

Fiji’s health facilities are almost overwhelmed as cases surge.

Get the latest updates here, as well as maps and a vaccine tracker.

Vaccines show early success against Delta

Scientists are trying to determine how vaccines work on Delta, but determining the effectiveness of a vaccine in the real world is tricky business.

In clinical trials it’s relatively easy to measure how well vaccines work, because groups of vaccinated people can be compared directly to a control group of unvaccinated people. But once vaccines enter the real world, scientists no longer have a control group, and other factors may determine if someone gets sick. People who forgo vaccination, for example, may be more likely to put themselves in situations where they could become infected.

This issue is at the heart of a number of new studies that have tracked the effectiveness of vaccines in protecting against the Delta variant. My colleague Carl Zimmer reports that researchers across the world are running experiments, and coming up with different answers.

In Britain, researchers reported in May that two doses of the Pfizer vaccine had an effectiveness of 88 percent against symptomatic disease from Delta. A June study from Scotland concluded that the vaccine was 79 percent effective against the variant. On Saturday, a team of researchers in Canada pegged its effectiveness at 87 percent. And on Monday, Israel’s Ministry of Health announced that the effectiveness of the Pfizer vaccine was only 64 percent against all coronavirus infections, down from about 95 percent in May.

Vaccine experts say that while this range of numbers may seem confusing, it’s actually normal, because it’s hard for a single study to accurately pinpoint the effectiveness of a vaccine.

“We just have to take everything together as little pieces of a puzzle, and not put too much weight on any one number,” said Natalie Dean, a biostatistician at Emory University.

While the 64 percent effectiveness found in the Israel study may seem worrying, there may be a few explanations for the number. The country requires testing of everyone who comes into contact with someone who has Covid-19. This could mean that Israel is spotting more asymptomatic cases in vaccinated people than other places, bringing their reported effectiveness rate down.

The researchers also did not match vaccinated individuals to unvaccinated counterparts in an attempt to simulate the control group, something that scientists often go to great lengths to do and that might have helped them rule out other factors.

Fortunately, all the studies so far agree that most Covid-19 vaccines are very effective at keeping people out of the hospital and have generally protected against the Delta variant. Israel’s Ministry of Health estimated that the Pfizer-BioNTech vaccine is about 93 percent effective in preventing serious illness and hospitalization.

But to truly determine how well vaccines work against the Delta variant, more studies from more countries are required.

“If there are five studies with one outcome and one study with another, I think one can conclude that the five are probably more likely to be correct than the one,” said Marc Lipsitch, an epidemiologist at the Harvard T.H. Chan School of Health.

Your Fitbit can track Covid’s effects

One in five Americans uses a wearable fitness tracker, many of which collect data on heart rates, body temperature, sleep and more. Throughout the pandemic, researchers have looked to devices like Fitbits or Apple Watches to understand Covid-19.

Early on, scientists studied whether the devices could detect early symptoms or predict a diagnosis. Now, researchers have used data to examine recovery and long-term effects of the disease.

In a new paper, researchers studying Fitbit data reported that people recovering from Covid-19 displayed behavioral and physiological changes that lasted longer than those with other respiratory illnesses.

Both groups slept more and walked less after they got sick, and their resting heart rates rose. But these changes were significantly more pronounced in people with Covid-19.

The scientists also found that about nine days after participants with Covid first began reporting symptoms, their heart rates dropped. After this dip, which was not observed in those with other illnesses, their heart rates rose again and remained elevated for months.

It took 79 days, on average, for those participants’ resting heart rates to return to normal. For those who had other respiratory illnesses, it took only four.

The Delta variant

New York City is scaling back Covid-19 monitoring just as Delta has become the predominant variant in the city.

Nevada is seeing a rise in cases and hospitalizations connected to low vaccination rates and the variant.

The variant is driving a surge of cases in Indonesia, which on Wednesday recorded nearly double the number of Covid deaths from two days ago, The Wall Street Journal reports.

The variant has driven a surge in cases among young, unvaccinated people in Spain, which now has the highest rate of cases in mainland Europe, The Financial Times reports.

What else we’re following

More than 125 people who attended a religious camp in Texas tested positive, and hundreds more were exposed.

Public health experts say the Biden administration is not being aggressive enough in its efforts to vaccinate the country.

Heathrow, London’s largest airport, will trial fast-track lanes for the vaccinated, the BBC reports.

New York City hosted a ticker-tape parade for its essential workers, but several unions declined to participate, citing low pay and a contract dispute.

The Economist designed a “normalcy index” to track how much life looks like it did in 2019 in places across the globe.

Downtown office districts in the U.S. have long been vulnerable to market fluctuations. Covid-19 just made it worse.

Start-ups are cashing in on the hybrid future of work.

Golf players and coaches worked remotely throughout the pandemic, practicing swings over video conference. (Weirdly, it seems to have worked.)

Some Wisconsin zoos plan to vaccinate animals vulnerable to the disease with an experimental vaccine, Wisconsin Public Radio reports.

What you’re doing

In the South African summer of 1956, when I was 19 months old, I got polio. My parents were avid and vocal anti-vaxxers. Now in this pandemic, I am appalled and insulted when people spout unsubstantiated anti-vaxxing theories. No one in their right mind would want to have foreign material injected into them if there was an alternative. There is no other alternative. The problem is not the lockdown, the vaccine or whatever else you may wish to entertain. The problem is this virus. The I.C.U. data in countries that have had aggressive vaccination policies is now indisputable: The vast majority of deaths in hospitalized Covid patients are in the unvaccinated.

— Analee Milner, Johannesburg

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