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Immunizing a nation is hard. With countries that made early gains in their mass vaccination programs now hitting hurdles as those programs drag on, governments are having to face an awkward question: What if the early part was the easy part?

That question looms large in the United States. On Tuesday, the White House admitted that the country would not reach the Biden administration’s much-hyped goal of getting at least one vaccine shot to 70 percent of eligible adults by July 4.

As The Washington Post’s Dan Diamond reported, falling daily vaccination rates had made this announcement a forgone conclusion. “The United States had fallen behind Biden’s goal as vaccinations slowed, particularly in the South and Midwest,” Diamond wrote.

“Health officials have struggled to persuade younger Americans to get vaccinated despite lotteries, gifts and other promotions,” he added, pointing toward surveys that “find that nearly one-third of Americans have no immediate plan to get vaccinated.”

Other nations are catching up. The European Union significantly ramped up its vaccination program in May, with daily doses administered peaking at around 4 million this month — well above the daily amount currently being administered by the United States.

Canada, a U.S. neighbor that also had a slow start, is closing the gap, too. Sixty-seven percent of Canadians are at least partially vaccinated. The United States no longer holds the record for most doses given out, either, with China now well ahead after vaccinating more than 1 billion people.

But focusing on raw numbers misses the complexity of this middle period of vaccination drives. Other countries are also likely to see their daily vaccination rates fall as they face problems with hesitancy, variants and supply.

Polling from the European Union shows that over a quarter of residents of the bloc say they are unlikely to take a vaccine. As in the United States, pockets of vaccine hesitancy threaten overall progress: In Bulgaria, where the survey found vaccine hesitancy was highest, 61 percent said they were unlikely to get inoculated.

Vaccine hesitancy is a continuing global problem. It has slowed Russia’s once-ambitious rollout to a crawl, prompting a spokesperson for Russian President Vladimir Putin to warn Tuesday that “discrimination will become inevitable” in the workplace for those who refuse to get vaccinated.

In India, where vaccine uptake continues to lag despite a devastating recent surge in the virus, hesitancy is one factor behind the country’s slow rollout.

“We have to convince people, go door to door, and rely on people who have taken the vaccine to spread the word,” Yogesh Kalkonde, a public health doctor in Gadchiroli, a tribal area in the western state of Maharashtra, told the Associated Press. “It’s an extremely slow process.”

Even where vaccination campaigns got off the ground well, achieving full normalcy is not easy. Britain had one of the most successful vaccination programs of any major economy, but it has had to stall its reopening due to the threat of the delta variant.

British researchers such as Oliver Johnson, a professor of information theory at Bristol University, have said the delta variant’s fast-spreading characteristics and the vaccines’ somewhat weakened ability to fight it show that achieving herd immunity could require 85 percent of the population to either have been vaccinated or to have immunity due to prior infection.

Without an even higher percentage of eligible adults being vaccinated, reaching herd immunity may require vaccinating teenagers, Johnson tweeted Wednesday, noting that such a tactic was not “consequence free.”

In Israel, which also has been billed as a success story, authorities are now wondering if they have to do precisely that. A spate of new cases in schools and concerns about the spread of delta have led to a new push to vaccinate teenagers, Reuters reported this week.

So far, fewer than 4 percent of 12-to-15-year-olds have been vaccinated since they became eligible this month, according to Israel Health Ministry data, while roughly three-quarters of adults have been vaccinated.

“Vaccinate your children,” Prime Minister Naftali Bennett said in televised remarks.

Israel is primarily using the Pfizer vaccine, which has a high efficacy even against the delta variant. But many countries are using less-effective vaccines, largely untested against delta. Countries including Bahrain, Mongolia and the Seychelles that have relied on Sinopharm and Sinovac, the lower efficacy Chinese-made vaccines, are now seeing significant waves of infection.

This may be one reason that China, despite its record-setting vaccination drive, is not keen to reopen soon: Beijing plans to keep pandemic border restrictions in place for at least another year, the Wall Street Journal reported Tuesday.

None of this is to say that the vaccines are failing. In fact, quite the opposite — vaccines still provide substantial protection against the virus. But rising cases in relatively highly vaccinated countries show that developing vaccines or even starting a successful vaccination program marks only the beginning steps of a long, slow and logistically difficult process.

The alternative, however, is worse. Coronavirus cases are rising across Africa, a continent that has so far been mostly spared from the worst outbreaks but still suffers from a severe lack of vaccine doses.

In Kenya, officials say the delta variant is one reason for a rise in cases. “The India example is not lost to us,” Boaz Otieno Nyunya, a county-level executive for health and sanitation, told the New York Times.

South America has become the world’s coronavirus hot spot in recent weeks, in large part due to limited access to vaccines across much of the continent. It’s resulted in a wave of death not seen in any other region.

“Paraguay, Suriname, Argentina, Uruguay, Colombia, Brazil and Peru are suffering — in that order — a silent decimation by Covid unlike that anywhere else in the world,” a team of reporters from the Guardian wrote this week. “Even in seventh-placed Peru, the number of deaths per million stands at 9.12 — more than three times the figure in India.”

These regional outbreaks carry with them global risks, slowing recoveries and raising the prospect of variants more worrying than delta. But efforts to share vaccine doses, such as the U.N.-backed Covax, have struggled, with officials saying this week that doses were running out in many parts of the world.

There has been widespread criticism of Covax, including by those sympathetic to its vision. But the struggles of vaccination campaigns at the national level only underscore the necessity of this worldwide effort.