The C.D.C. says there is limited data on the vaccine's effectiveness when shots are separated by more than six weeks, although some countries, including Britain and Canada, are giving shots with a gap of up to three or four months.

College students pose a particular challenge. Many recently became eligible to be vaccinated and are getting their first shots, but they will have left campus by the time they are due for their second doses.

In many cases, vaccine providers had canceled second-dose appointments because of bad winter weather.

There are rare cases in which people are supposed to forgo the second shot, such as if they had an allergic reaction after their first shot.

The stakes are high because there is only one vaccine authorized in the United States that is given as a single shot. The use of that vaccine, made by Johnson & Johnson, was paused this month after it was linked to a very rare but serious side effect involving blood clotting. Federal health officials on Friday recommended restarting use of the vaccine, but the combination of the safety scare and ongoing production problems is likely to make that vaccine a viable option for fewer people.

The C.D.C.'s count of missed second doses is through April 9. It covers only people who got a first Moderna dose by March 7 or a first Pfizer dose by March 14.

In some cases, problems with shipments or scheduling may be playing a role in people missing their second doses. Some vaccine providers have had to cancel appointments because they did not receive expected vaccine deliveries. People have also reported having their second-dose appointments canceled or showing up only to find out that there were no doses available of the brand they needed.

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Israeli media reported that an estimated 700 Palestinians have been protesting near the closed Karni crossing between Israel and the Gaza Strip, with the IDF reportedly using flares and stun grenades to contain people without giving them a chance to get close to the security fence.

Video allegedly showing the IDF firing flares near the security fence was shared on Twitter.

Mass demonstrations were reported throughout the Gaza Strip, as thousands of Gazans hit the streets to express support for the Palestinians who live in Jerusalem. Photos and video said to have been taken in different parts of Gaza were shared on social media.

Protests also took place across the West Bank, where people marched to Israeli checkpoints, including Jenin's Jalameh checkpoint, Nablus's Huwarra checkpoint and Hebron's Bab al-Zawiyeh.

Around one hundred Palestinians were reported protesting near the Qalandiya checkpoint between Jerusalem and Ramallah, the de facto administrative capital of the Palestinian National Authority (PNA). According to The Times of Israel, citing a spokesperson for the border police, "rioters" were using firebombs, stones, and fireworks, while police were responding with "riot dispersal means".

Meanwhile, a new series of encounters between Palestinians and Israeli police has started at the Damascus Gate outside Jerusalem's Old City. According to the Palestinian Red Crescent, six Palestinians were hurt during clashes at the gate. According to The Times of Israel, two police offciers were injured during a rock-throwing incident in East Jerusalem and one was hit in the face in the Old City.

PNA President Mahmoud Abbas is set to chair a meeting of Fatah's Executive Committee on Sunday to discuss the ongoing dynamics.

"This meeting will discuss the dangerous events in Jerusalem, and to review the movement's preparations for the electoral process, especially in the context of the Palestinian leadership's insistence that there will be no elections without Jerusalem," Fatah Central Committee member Sabri Saydam said, cited by The Times of Israel.

Palestinians are preparing for their first general elections in 15 years. The parliament elections in both the West Bank and the Gaza Strip are set to take place on May 22, with the presidential election scheduled for July 31.

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