Jeremy Laurance: The offer of a holiday is a considerable lure

When I mentioned the Mexican drug trial run by Intercell to my son, his response was ecstatic. "Yessss!" he exclaimed. "I've always wanted to go to Mexico." Within seconds, he was on his laptop.

It is easy to see why Intercell's chief executive is so confident that recruitment to the trial will "not be an issue". He knows the offer of a free holiday to an exotic destination, flights and hotel included, is a considerable lure. I even wondered about applying myself.

But suppose volunteers were instead offered €1,600 (the price equivalent) to spend a couple of days in a lab at Northwick Park Hospital testing the new vaccine. I wonder how many would come forward.

The infamous "Elephant Man" drug trial at Northwick Park in 2006, which left six men fighting for their lives, cast a dark cloud over drug research. Offering yourself as a human guinea pig, previously seen as an easy way for students to supplement meagre incomes, was transformed in the public mind into a deadly game of chance.

The Mexican trial is nothing like the Northwick Park one. The vaccine being tested has already been tried on human volunteers in the US and found to be safe and effective. Nevertheless, there is always the possibility that in a larger number of subjects there might be someone who has an adverse reaction.

The offer of a holiday might appear gratuitous in most drug trials, but in this one there is a clear rationale. A vaccine against travellers' diarrhoea must be tested in parts of the world where the problem is endemic.

Safety regulations for UK drug trials have been strengthened in the past five years. Volunteers can be assured that no unnecessary risks are run. But even the best-regulated studies can have unexpected outcomes – as Northwick Park proved.