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This is the Coronavirus Briefing, an informed guide to the pandemic. Sign up here to get this newsletter in your inbox.

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Credit…The New York Times

One Covid case prompted closures in a city of 10 million in China.

Construction workers in Melbourne, Australia, are protesting mandatory vaccinations.

A gas station attendant was killed in Germany after he told a customer to mask up.

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

J. & J.’s booster bump

Over the last few weeks, millions of people who have received the Johnson & Johnson vaccine have been in limbo as they wait for guidance about whether they’ll need booster shots — and to learn what protection, if any, an extra dose might offer. Today, they’re finally getting some clarity.

Johnson & Johnson said this morning that a second dose of its vaccine substantially boosted protection against Covid-19. In a clinical trial, researchers found that two doses of the vaccine offered 94 percent efficacy against mild to severe Covid in the U.S., up from 74 percent with a single shot. And two shots showed 100 percent efficacy against severe disease (although that estimate had a wide range of uncertainty).

J. & J. vaccine recipients should also take heart in new data that suggest that one dose of the shot continues to offer strong protection over time.

The doses from Pfizer and Moderna offer high levels of initial protection that seem to diminish slightly over several months. But a recent study showed that up to five months after vaccination, the J. & J. vaccine’s effectiveness against hospitalization remained steady at about 81 percent.

As for the two-dose regimen, a new clinical trial found that the second shot boosted antibody levels in volunteers four times as high as that of only one shot, which translated into stronger protection. Some preliminary studies suggest that higher levels of antibodies against the coronavirus produce higher levels of protection against Covid.

Johnson & Johnson said it had submitted the data to the F.D.A. for review. Last week, an advisory committee to the F.D.A. recommended booster shots for Pfizer recipients who are at least 65 years old or at risk of Covid.

The benefits of solo masking

In The Times’s “Ask Well” section, reporters and experts answer readers’ health questions. (Is carbonated water just as healthy as regular water? Why does coffee sometimes make me tired? Can vaccinated people develop long Covid after a breakthrough infection?)

Today, my colleague Tara Parker-Pope took on a reader question I’ve often wondered about: “If I’m the only person wearing a mask in a store or other indoor location, am I really protected from infection?”

It’s true that masks work best when everyone in the room is wearing one. That’s because when an infected person wears a mask, a large percentage of their exhaled infectious particles are trapped, stopping viral spread at the source. And when fewer viral particles are floating around the room, the masks that others are wearing are more likely to block those particles that have escaped.

But there is also plenty of evidence showing that masks protect wearers even when others around them are mask-free. The amount of protection depends on the quality of the mask and how well it fits.

One study from the C.D.C. found that a standard surgical mask protected the wearer from only about 7.5 percent of the particles generated by a simulated cough. But knotting the loops and tucking in the sides of the medical mask reduced exposure by nearly 65 percent. (Watch this video to see the “knot and tuck” method.) Covering the surgical mask with a cloth mask, a technique known as double masking, reduced exposure to the simulated cough particles by 83 percent.

A study from Tokyo tested how well different types of masks protected wearers from actual coronavirus particles. The study showed that even a simple cotton mask offered some protection (17 to 27 percent). Medical masks performed better, including a surgical mask (47 to 50 percent protection); a loosefitting N95 (57 to 86 percent protection); and a tightly sealed N95 (79 to 90 percent protection).

Given that the Delta variant is far more contagious than other variants, experts recommend wearing the highest-quality mask possible when you can’t keep your distance or aren’t outdoors — especially when nobody around you is masking up.

Have you persuaded someone to be vaccinated?

Persuading hesitant people to receive the vaccine is not easy.

For some people, it’s enough to have a person in authority, like a doctor or health care worker, explain the benefits and give assurances that doses are safe and effective. But for others, that’s not enough — and that approach can even backfire.

So then, how can you persuade someone to receive a vaccine?

We’re hoping you can help us answer that question. Have you persuaded a friend, family member or someone else? We’d love to hear how you did it.

We’re collecting responses for an upcoming edition of this newsletter. If you’d like to participate, you can fill out this form.

What else we’re following

President Biden calls for global unity against common threats. The U.N. secretary general urged nations to address a “cascade of crises.”

Unvaccinated and defiant, President Jair Bolsonaro of Brazil pushed back against criticism in his U.N. speech.

The Organization for Economic Cooperation and Development said that uneven global vaccination is threatening the economic recovery.

The Biden administration is looking to help more rural Americans find health care.

For families in Europe, the end of the U.S. travel ban is bringing relief.

Greece benefited from opening early to tourists.

What you’re doing

This summer, I was very excited to spend three and a half weeks at sleepaway camp after it was canceled last year. Before I left for camp, Covid cases were going down and spirits were rising all around me. I stopped wearing a mask everywhere I went, but started bringing it with me for the subway and other high-risk situations. I hugged vaccinated family members who I hadn’t seen in months. Then, I went to camp where I was in a Covid-free bubble with no pandemic on my mind for 25 days. When I returned, my mom told me to avoid the news. In just a couple minutes, I found out about Afghanistan, our growing climate crisis, and that Covid case curve rising again due to the vicious Delta variant. I pulled out my box of masks and wondered if the world before I left had been a dream or reality.

— Maddi, 16, New York, N.Y.

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