Up to a third of HIV infections in European gay men may have come from another country

Recent infections also more likely to have crossed borders
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A study (Frentz) that looked at genetic similarities in the HIV from recently diagnosed people in 24 European countries found that among ‘clusters’ of closely related viruses (which indicate networks of transmission), a quarter of people who were in a cluster were connected to people diagnosed in other countries.

This was the particularly the case in gay men, where nearly a third (31%) of people in viral 'clusters' were connected to at least one diagnosis in another country. In contrast, only 14% of heterosexual people belonged to international clusters, indicating less mobility.

People who had been recently infected, and people with HIV subtype B, were also more likely to belong to a cluster indicating cross-border infections: but as these are both more associated with gay men than with other risk groups, only being gay remained significantly associated with belonging to an international cluster.


transmission cluster

By comparing the genetic sequence of the virus in different individuals, scientists can identify viruses that are closely related. A transmission cluster is a group of people who have similar strains of the virus, which suggests (but does not prove) HIV transmission between those individuals.


In HIV, different strains which can be grouped according to their genes. HIV-1 is classified into three ‘groups,’ M, N, and O. Most HIV-1 is in group M which is further divided into subtypes, A, B, C and D etc. Subtype B is most common in Europe and North America, whilst A, C and D are most important worldwide.

representative sample

Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).


Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).

risky behaviour

In HIV, refers to any behaviour or action that increases an individual’s probability of acquiring or transmitting HIV, such as having unprotected sex, having multiple partners or sharing drug injection equipment.

Since two-thirds of ‘clusters’ actually only comprised two people, this indicates that a significant proportion of Europeans diagnosed with HIV, and especially gay men, acquire their HIV either while abroad, or from someone from abroad. This phylogenetic study shows that risky sexual encounters among gay men while travelling abroad, as seen in another large Euopean study, are indeed resulting in some cross-border HIV transmission.

On the other hand – as the researchers emphasised – the fact that a majority of connected infections, especially among heterosexuals and people with non-B HIV subtype, were in sole-country clusters, also indicates that we may need to revise assumptions that immigration from high-prevalence countries is still the biggest contributor to infections among heterosexual people.     

The SPREAD study: patient characteristics

This study is part of an EU-funded research collaboration called the SPREAD Programme, which has been analysing the genetic makeup of HIV from recently diagnosed people in 25 European countries (samples taken from 2002 onwards).

In this study, the SPREAD researchers genetically analysed samples from 4260 recently diagnosed people, with the largest numbers from Germany, Spain, Sweden, Belgium, the Czech Republic and Denmark. Fifty-five per cent of participants gave their origin as western Europe, 21% eastern Europe, and 11% sub-Saharan Africa. Nearly half (48%) said they most likely acquired HIV from gay sex, 35% from heterosexual sex and 8% from injecting drugs.  Seventy per cent of people were living in their country of origin; in Israel, Sweden, and Norway, 50% or more of the people diagnosed were immigrants from outside Europe, with Ethiopia, Zimbabwe and Thailand the most common countries of origin. 

The mean viral load in these people was 63,000 copies/ml and their mean CD4 count 363 cells/mm3. Twenty-nine per cent of infections were acquired less than a year ago, though in two-thirds of cases the duration of infection was unknown. Nine per cent of the samples analysed were resistant to at least one HIV drug.  

Cautions and caveats

Before commenting on clusters, it is important to mention that this study is not representative of all infections throughout western and central Europe. The UK and France are not part of the SPREAD programme, and neither are Switzerland, Hungary and several Balkan countries. It is important also to emphasise that SPREAD only looks at a sample of HIV infections in each country and that, in some countries, risk groups were over- or under-represented. Some countries, such as Poland and Cyprus, do not routinely collect data on which risk group diagnosed people belong to.

Finally, only 31% of HIV viruses were closely related to any other infection, so the majority of infections were not in ‘clusters’ and one cannot say anything about where they might come from. The definition of being in a ‘cluster’ is that a virus was more than 98% genetically similar to at least one other virus analysed by SPREAD. As the researchers say, they might have detected many more clusters given whole-country databases to analyse.

Connected infections: findings

Nonetheless, some interesting data emerged from the 457 clusters analysed, and the 1330 people who belonged to those clusters. Seventeen per cent of clusters featured people diagnosed in more than one country and these international clusters tended to be larger, containing 26% of diagnosed people.

Four in ten of these international clusters featured people only from directly neighbouring countries, but the other 60% included people diagnosed in countries not sharing a common border. The largest cluster comprised 28 people with closely similar virus who were diagnosed in Slovakia, the Czech Republic and Italy. There was also a large German/Spanish cluster.

Patients found to be in clusters were more likely to be gay men. Sixty-three per cent of people in clusters were gay versus 41% not in clusters They were also more likely to have subtype B virus, the predominant type in native Europeans (82% in clusters, 59% not in clusters), and to have been infected in the last year – 39% of people in clusters had been recently infected versus 24% not in clusters

Thirty-one per cent of gay men in clusters were in an international one and 29% of people recently infected.

In contrast only 15% of injecting drug users, 14% of heterosexual people and 13% of people with a non-B subtype of HIV were in an international cluster.


This study confirms findings from a number of other national and international studies. On the one hand, studies from Belgium and Switzerland show that HIV rarely crosses between risk groups, and a study from the Netherlands found that HIV from African and Caribbean immigrants was not crossing into the general population. As the researchers comment about this study, “The small number of migrants, their relatively moderate sexual risk behaviour and low mixing with Dutch heterosexuals” means that spread of non-B-type HIV into the heterosexual population is hardly happening.

Even in gay men, 70% of clustered infections were solely in one country. However, it is also notable that gay men were twice as likely as heterosexual people to be members of internationally connected clusters of infection, and also more likely to have been recently infected. (In this connection, it is also interesting that the proportion of people [in clusters or not] who were recently infected increased from 33% of diagnoses in 2002 to 48% in 2007, though this could just as well indicate increased testing rates as an increased rate of infection.)

An analysis of the large European EMIS Study of men who have sex with men, presented at the International AIDS Conference in Washington last year (Fernández-Dávila), found that a substantial number of gay men in Europe had their last high-risk sex while not in their own country, with Spain and Berlin in Germany the most frequently cited locations (and Paris and London, not included in SPREAD, coming in as the third and fourth most popular destinations).

This study suggests both that settled immigrant communities may need better information about HIV risk in their own community but also that gay men may need better information about infection risk whilst traveling abroad.


Frentz D et al. Limited cross-border infections in patients newly diagnosed with HIV in Europe. Retrovirology 10(36), 2013.

Fernández-Dávila P et al. Mobile men who have sex with men: an exploration in European residents of sexual risk taking while travelling abroad. 19th International AIDS Conference, Washington, abstract THPE373, 2012.