In November, Spain's Prime Minister Pedro Sanchez made a promise before the start of his country's vaccination campaign. 

"Spain will have three-quarters of its adult population vaccinated by the end of the summer," he said. 

The fact that Spain has now caught up with or overtaken both the UK and the U.S. in terms of the population fully immunized reflects the fact the country is likely to reach that target.

In a tweet, Sanchez said: "Spain has almost 55 percent of the population fully vaccinated. @OurWorldInData returns to pick up the impressive rate of vaccination at which our country is progressing. Thanks to all the people who make this great success possible every day. We continue!"

Sanchez was referring to a study by the scientific online publication Our World In Data. As of July 24, the study found that Spain had fully vaccinated 54.3 percent of its population.

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In Europe, that puts it above the UK, Germany, Italy, and France and getting close to Hungary as the country with the highest proportion of fully vaccinated adults. 

One reason for Spain's successful immunization program could be down to the country's relatively high support for vaccination, with more than 91 percent of people aged over 40 opting to receive a jab.  

And what makes the figures even more impressive is the fact that from January until the end of April, Spain had fully vaccinated just 10 percent of its adult population.

Maria Luisa Domingez works for SUMMA 112 - Emergency Services, the organization in charge of Madrid's vaccine roll-out. 

She told CGTN Europe: "The vaccination strategy used by Spain was to focus on full vaccinations, to ensure we could deliver the second dose quickly, instead of trying to vaccinate everyone with the first dose.  

"So just 21 days later, we administer the second jab to fully vaccinate the individuals."

Public confidence in vaccines

CGTN Europe went to one of Madrid's main vaccination centers, the WiZink complex. 

Many of the people receiving their jabs said they felt confident the vaccines were safe and were working.

"I think people trust it now, they have more faith in this. We have to believe in the science and keep moving forward," one man told CGTN Europe.

Another man added: "I think we trust in this for sure. You just have to look at the numbers of people already vaccinated. 

"Everyone has changed their mind about this. It's all changed. Back at the start, there was much more skepticism and now it's a lot less. We can see it's working."

However, the vaccines have not been enough to stop the recent surge in infections.

The latest figures from the health ministry showing the 14-day infection rate is now 700 per 100,000 people.

That's among the highest in Europe and that's a statistic Spain's prime minister may be less eager to tweet about.