The impact of immigration on the NHS has long been a contentious topic and the Queen's Speech last week, which outlined plans to restrict migrants' access to NHS care, has thrust it into the headlines once again. Yet hard data on the issue have sometimes been thin on the ground.

In 2011, the Nuffield Trust published an analysis that looked at how often international migrants to England use hospital care in comparison with English-born people.

To our knowledge, few studies had been able to look at the issue. In Nuffield Trust style, we used national hospital administrative data sets in order to make some progress in a national study of resource allocation.

We faced some difficulties, as immigration status was not recorded in the administrative data that we had access to. However, we were still able to identify a group of probable migrants by linking the hospital data with anonymous data on GP registrations.

The analysis raises the important question about why recent migrants to England seem to use hospitals less often than English-born people

After assembling the linked data, we focussed on people who registered with a GP in England when they were aged 15 or over, but who did not have a previous registration with a GP. We thought this group would be overwhelmingly made up of migrants from outside of England, assuming that most English-born people register with a GP before the age of 15.

The method is not perfect, but we identified 550,000 probable immigrants registering in the 2003/4 year. This rose to 583,000 for 2004/5 and to 625,000 for 2005/6.

We found that the group of probable immigrants used hospital care relatively rarely – in fact, admission rates were around half that of English-born people of the same age and sex. Rates were most similar for obstetrics and neonatal care, but even then they were still lower for immigrant women of childbearing age than for similarly-aged English-born women.

The patterns of lower admission rates among international migrants to England persisted for at least three years after their initial registration with a GP. And they were similar for three different cohorts of international migrants that we studied (arrivals in 2003/4, 2004/5 and 2005/6) – so before and after the Eastwards expansion of the European Union in May 2004.

Since our analysis was published, the findings have been borne out in a number of other analyses across Europe – including in Norway and Germany.

Interestingly, some studies have found a higher level of GP use among immigrants than native-born people (for example, in Spain). We were not able to look at this for England, however.

The analysis does not tell us everything we would like to know about migration and health care.

For instance, our data were specific to England, so we could not distinguish between migrants from outside of the UK and migrants from Scotland, Wales and Northern Ireland. Further, we were not able to distinguish between international migrants who become 'ordinarily resident' in the UK (and so are entitled to free health care) and those who are in the UK on a more temporary basis.

Despite the limitations, the analysis does raise the important question about why recent migrants to England seem to use hospitals less often than English-born people. There are a number of possible explanations.

One, suggested by previous studies, is that people who migrate are typically healthier than average, perhaps because healthier people are more likely to be physically capable to move and find work.

Another possible explanation is that some international immigrants return to their country of origin when they need health care. This might be because they trust and understand health services in their country of origin better than in the UK, or because when people become very ill they choose to be near family members (or perhaps can no longer support themselves in the UK).

Finally, it is possible that some international migrants find it harder to access services than English-born people.

Regardless of the explanations, the analysis suggests that the overall pattern of health service use for migrant populations is one of particularly low levels of hospital admission. As the proposed Bill is debated in Parliament and elsewhere, the increasing evidence base about migrant health should be borne in mind.

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