Shares in the UK’s Allergy Therapeutics have taken a hammering on the news that US regulators have put a block on clinical trials of the firm’s new hayfever vaccine.

The company said that the US Food and Drug Administration has placed a hold on two Phase III studies of its lead product Pollinex Quattro following a report of a rare adverse event, which the doctor involved in the study said was “possibly related to the treatment. The patient in question displayed neurological symptoms such as numbness and weakness, but made a full recovery after being treated in hospital.

Allergy has been taken aback by the decision but added that it is collaborating fully with the FDA and has provided extensive data,

“including independent expert assessment”. The firm added that it has also reviewed all the data concerning the adverse event, and considers “that it is unlikely that the event was caused by the study drug”. The company has three versions of Pollinex Quattro under development, and the two halted are for hay fever caused by grass and ragweed. A tree pollen study is at the Phase II stage.

Chief executive Keith Carter was diplomacy personified, saying that “the FDA has a difficult job to do in minimising risk to patients, both on clinical trials and from marketed pharmaceuticals [and] it is normal that they should exercise caution”. However he stressed that “the evidence strongly supports the view that the event was unlikely to have been caused by Pollinex Quattro and we are seeking urgent clarification of the FDA's position”.

The product has already been given on a named-patient basis to around 100,000 hayfever sufferers all over Europe in recent years and no cases similar to the one the FDA has cited have ever been reported. Speaking on a conference call, Mr Carter noted that the clinical hold has to be lifted within a week, or one of its trials could be compromised, saying “we need to know where FDA is coming from. It is possible they have other data not available to us”.

Allergy had been hoping to launch Pollinex Quattro in 2009 in the USA and the fact that this may now be delayed, rather than any fears about the safety or efficacy of the vaccine, that frightened investors. The share price ended the day down almost 22% to £0.93.