What happened? Here are six lessons learned from Israel's experience — and one looming question for the future of the pandemic.

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Israel is a country that jumped out early in making COVID-19 vaccines widely available. But it still didn't vaccinate enough people to hold off the spread of the delta variant. And now it's once again dealing with high rates of infection. So what might that tell us about the effect of the virus in a country where most are vaccinated? NPR's Daniel Estrin reports from Jerusalem.

DANIEL ESTRIN, BYLINE: In late March, Israel was the first to fully vaccinate most of its citizens. Infections waned. Restrictions were lifted. But then, Israel discovered something other countries would later see. Siegal Sadetzki, former Israeli public health director in Israel.

SIEGAL SADETZKI: People were vaccinated first, and the immunity most probably goes down a bit.

ESTRIN: The Israeli government and Pfizer say their data show a dip in the vaccine's protection around six months after the second shot. For Israel, that came around the same time that the delta variant arrived.

SADETZKI: The most influential event was so many people who went abroad in the summer on vacations and brought the delta variant very, very quickly to Israel.

ESTRIN: Serious cases have spiked. The vaccine still prevents most infections, and infections are mostly mild if you're vaccinated. But while the country jumped out ahead on vaccines, only 58% are fully vaccinated. And experts say that's not nearly high enough.

ERAN SEGAL: We have a very large fraction of our population who are paying the price for a small fraction of the population who did not go to get the vaccine.

ESTRIN: Scientist Eran Segal advises the Israeli government on COVID. He says 1.1 million eligible citizens, largely under the age of 20, have not been vaccinated. And they kept spreading the virus while the country remained open for business with few serious restrictions.

SEGAL: If you don't put in very strict measures, which the government has not done until now, then basically you are letting the virus spread. And what that means is that more and more people will get infected, and that will lead to mass infection, which is exactly what we are seeing now.

ESTRIN: With daily cases now nearly 10 times what they were in mid-July, Israel is trying to slow the wave without resorting to a new lockdown. It's restricting gatherings, boosting hospital staff and pushing vaccinations. And it was the first country to give a third shot of the Pfizer vaccine nationwide. I saw this in Jerusalem.

This Cinema City movie theater complex is teeming with people - lots of young people - here to see movies. And right across from the box office, a vaccination station, where dozens of people are waiting to get their booster shots, hoping maybe that will give them more protection.

UNIDENTIFIED PERSON: (Speaking non-English language).

ESTRIN: People lining up for shots in Israel know they're being watched around the world, just like when Israel was early with the first round of shots.

ETTI BEN YAAKOV: That make the test of us. But in the first, it was the same. So I don't feel it's something wrong. I think it's good.

ESTRIN: That's Etti Ben Yaakov, one of more than 1 million Israelis who got a booster in the last three weeks.

BEN YAAKOV: I know that the corona not will be in the world.

ESTRIN: Won't go away.

BEN YAAKOV: No. It will be - we will have to live with the corona.

ESTRIN: U.S. officials looked at Israeli data when it decided to start booster shots across America next month. They'll also be looking at how the boosters work in Israel for a glimpse at what we might see in the U.S. further down the road. Daniel Estrin, NPR News, Jerusalem.

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Correction

An earlier version of this story misspelled the Weizmann Institute of Science as the Weitzmann Institute of Science.