Swine Flu Jab - Protection for a child if only had the first half dose?

Hi

Does anyone know if a child receives any protection from swine flu if they've only received the first half of a full dose vaccination? My 3 year old son had his first half last week, but I've been told I need to wait another 3 weeks for him to get the 2nd half and my doctor's surgery hasn't got any further clinics planned at the moment until new supplies have been received.

Thanks!