Trying to figure out how COVID-19 vaccination is going in the U.S. is a lot harder than you might think.

There are, on the one hand, all those stories about problems across the country, whether its pharmacies throwing away doses because they cant find people that fit eligibility criteria or its senior citizens struggling with online questionnaires to get appointments.

Theres also the fact that the coronavirus shots are going into arms a lot more slowly than the Trump administration promised they would. Officials had boasted of potentially 20 million vaccinations by the end of 2020. But only 14 million doses actually shipped by that deadline and only a small fraction of those shots actually went into peoples arms.

Still, its possible that President Donald Trump s goal was never realistic, given the challenges of launching such a massive program in a country with such a notoriously underfunded public sector. The pace is picking up now and, in a few parts of the country, states are well on their way to vaccinating the highest priority groups.

Also, any assessment of U.S. performance has to take into account how other countries are doing. It turns out the U.S. is faring pretty well, relatively speaking. In fact, shots are getting into arms faster than in most of Western Europe, at least according to the available data.

So whats a fair assessment of progress to date? And what would it take to achieve something like herd immunity this summer, which feels like the best-case scenario right now?

Heres a quick rundown of what we know, what we dont, and what it might take to make things go more quickly:

How Many Vaccines Have Been Administered?

As of Wednesday, nearly 11 million shots had been administered across the U.S., according to tracking by Bloomberg based on federal and state data. That is less than half of the total shots distributed to the states, which is obviously a big problem and has been ever since the rollout began.

One reason is that the Pfizer and Moderna shots, which are the two currently available in the U.S., require two doses. State and federal officials were holding back half the supply in order to make sure that everybody who got a first dose would also get a second one on the prescribed schedule. (Thats three weeks later for Pfizers, four weeks later for Modernas.)

Public health experts and eventually the Biden transition team called for changing that practice, on the theory that vaccine supply will probably be steady enough to provide second doses on time without holding any back now. The Trump administration apparently agreed and, just this week, called for releasing the second doses .

That change should make a difference. And the pace of vaccination was already increasing . There were 852,000 shots given on Tuesday, according to Bloombergs tally, which was one of the highest single-day totals yet.

Are Other Countries Doing Better?

Assessing the U.S. performance internationally depends on the context you choose or, more precisely, which countries are part of the comparison.

Among economically developed countries, the leader is Israel and its not even close. As of Tuesday, more than 1 in 5 Israelis had received at least one vaccine dose, according to data from Our World in Data . And the rolling seven-day average for daily doses in Israel was 0.75 per 100 people. The comparable figure for the U.S. was 0.19.

So Americans are getting vaccines a lot more slowly than their Israeli counterparts. At the same time, theyre getting them significantly more quickly than most of their European counterparts. Only the United Kingdom, where the rolling seven-day average was 0.29 on Tuesday, is putting more shots into arms per capita every day.

The international comparisons come with two very important asterisks. One is that not every country is tracking vaccine administration the same way or with the same accuracy. In many nations, including the U.S., data may be lagging behind actual shots.

The other asterisk is that different countries started vaccinating on different days because government approvals of vaccines didnt happen simultaneously. The U.K. started first, then came the U.S. and then, finally, the European Union. Adjusting for that factor, the performance of the U.S. comes closer to that of the U.K., but several European countries, like Denmark, Ireland and Italy, come close to or look better than the U.S.

Probably the fairest way to judge the U.S. relative to its peer countries is to say that it matches or exceeds the highest performing countries in Western Europe but, like those countries, hasnt had nearly the success that Israel has again, assuming that the data is roughly accurate.

What Can The U.S. Learn From Israel?

Israels success starts with its national health system , in which every single citizen belongs to one of four large, nonprofit health maintenance organizations. These HMOs, which are different from their American analogues in many respects, do not merely pay for care. They provide it directly. They also have cutting-edge data and communications systems in place.

We immediately set up a data system that identified which people to summon as a first priority, who was second priority, and who could wait. The whole thing took 10 minutes, Sigal Regev Rosenberg, chief executive of one HMO, told The Times of Israel .

Many other countries with national health insurance systems dont have that kind of capability, which is likely one reason that vaccination in countries like France and Germany has had a slower start. It may be no coincidence that the U.K., the European country administering shots the quickest, is also the one where, as in Israel, the government is heavily involved in the provision of care and not just its financing.

Another Israeli advantage is that the nations population is schooled, from a young age, in crisis response. That includes its Arab citizens, although vaccination rates for them are reportedly lower. The main reason for that, according to officials, is that Arab Israelis dont trust the vaccine as much.

(Vaccination in the occupied territories is a whole other story. Israel hasnt offered to provide shots there, according to media reports, and the Palestinian authorities who govern there havent asked.)

Israels small size may also be helping. With fewer than 8 million people living within a relatively thin strip of land, the national government can make and implement policy directly without a lot of delegation to lower-level governments, organizations and officials. Thats not really practical in countries with many tens of millions, let alone the 330 million who live in the U.S.

What Can The U.S. Learn From West Virginia?

The sheer size of the U.S. population helps explain why the Trump administration delegated not just actual vaccine delivery but also most of the planning for it to the states, which have then leaned heavily on local health departments. But that, in turn, helps explain why vaccine rollout has fallen so short of the initial goals.

Local health departments are overwhelmed, the states dont have extra money, and the federal government hasnt provided enough support .

This was never going to be easy, Jennifer Kates , senior vice president at the Henry J. Kaiser Family Foundation, told HuffPost. But the federal government basically punted the hard stuff to states who, left reeling from the ongoing impact of COVID-19, werent able to mount a robust effort in most cases. In addition, many states in turn punted the hard stuff to counties. At the end of the day, everyone is playing catch up and the stakes are very high.

Some states, naturally, have managed better than others. The three having the most success again, based on Bloombergs tracking of vaccines administered relative to population are West Virginia , North Dakota and South Dakota . Other places doing relatively well include Connecticut and Maine . Alabama is where vaccines are getting into arms the slowest. California is really struggling too.

That ordering is not what a lot of people would expect, especially when it comes to the top three. The Dakotas, in particular, have some of the highest COVID-19 death rates in the country, even though they didnt face major outbreaks until relatively late, by which time the American health care system had learned a great deal about how to prevent and treat the disease.

So what are these top three states doing right? Theres no single answer and, as with the international data, its possible some state reporting is incomplete or slow, making it hard to draw definitive conclusions. But a common factor seems to be their approach to nursing homes. Most states in the U.S. are relying heavily on CVS and Walgreens, which are the official partners of the Trump administrations Operation Warp Speed. But because CVS and Walgreens dont always have a presence in rural areas, those three states are relying more heavily on other providers to reach nursing homes.

West Virginia, in particular, is working exclusively through small, independently owned pharmacies. That turned out to be a smart move because the independent pharmacies already had strong working relationships with the long-term care facilities. And that enabled them to deal with logistical issues quickly, starting with the need to get consent from patients or (when patients have diminished capacity) their families.

When it got here, we already had pharmacies matched with long-term care facilities, so we were already ready to have vaccinators and pharmacists ready to go into those facilities and start providing first doses, Krista Capehart, director of regulation for the West Virginia Board of Pharmacy, told NPR .

CVS and Walgreens were much slower to get started. And slow penetration into nursing homes is a big reason that vaccination rates are lower in so many other states.

Its an important lesson, but not one thats likely to help much in the short term. The reality is that the big pharmacy chains now dominate the market, especially in more densely populated parts of the country. Thats a big reason why the Trump administration reached out to them.

And the big chains do offer a big upside: their massive scale. They have a large workforce of pharmacists already trained to give vaccines, appointment systems already in place, and physical facilities all over the country. Thats bound to help a lot in the coming months, as supply increases and many more people become eligible.

What Can The U.S. Learn From Connecticut?

Connecticut s success might be easier for other states to replicate. A big factor there was the discretion given to health care providers to deviate from eligibility criteria when the alternative was slowing down the process or discarding unused vaccines altogether.

Connecticut has emphasized equity, but also pragmatism in their rollout, said Howard Forman , a physician and public health professor at Yale University who has followed the vaccine rollout closely (and actually administered shots himself). They have provided sufficient flexibility to health systems to make sure that vaccines are getting into arms of health care workers without overly mandating (or penalizing) rules of engagement.

Flexible guidelines arent a panacea, but they do seem to help and most states are moving in that direction now, with encouragement from the federal government.

What Can The Biden Administration Do?

The next big challenge will come when supply ramps up, in late winter or early spring, and vaccines become available to a much larger swath of the public. At that point, the upper limit on vaccines will depend less on the amount of vaccine available and more on the ability to deliver it. The current plan is to use existing providers not just pharmacies, but also clinics, independent medical practices, and sometimes schools and employers and to supplement them with government-managed mass vaccination centers.

Those centers are already starting to be established across the country, everywhere from Detroit to rural Tennessee to Southern California. (Officials just started giving shots at Disneyland .) The incoming Biden administration has pledged to support even more of these centers and to secure yet more funding, above and beyond the $8 billion that was part of the COVID-19 relief package that Congress passed and Trump signed in December.

That $8 billion can give overwhelmed state and local authorities the ability to hire extra staff, both for planning and for administering the shots. It would have been great to get the funding months ago, when public health experts and officials were begging for it , warning of precisely the sorts of problems plaguing the U.S. effort now. But the money will still help a lot especially because the pace needs to pick up a lot more.

The Biden team has said that its goal is 100 million vaccine doses within the first 100 days. Thats not as audacious a goal as it seems, even though press reports suggest that members of the transition are worried about reaching it. The figure works out to a million a day, which isnt that far from where the vaccination effort is now. To reach herd immunity by summer, the pace probably needs to be twice as fast at least.

President-elect Joe Biden s advisers may simply be setting low expectations which, after four years of an administration that chronically overpromised, might not be the worst thing in the world. But the reality remains the same: Getting back to normalcy in 2021 will require doing much, much more than the U.S. is doing now.

CORRECTION: The original version of this article said the number of vaccinations by the end of 2020 was half of the 20 million target the Trump administration had cited in public statements. In fact, it was significantly lower than half perhaps as low as 3 million.