PIET ADEMA, DUTCH MINISTER OF AGRICULTURE

spread of the virus. The two vaccines which have proven effective are now moving on to field trials, which are expected to start this summer. Trials of vaccines for ducks, turkeys, and geese are taking place in other EU member states.

It is worth noting that the Dutch researchers had begun by investigating four vaccines last summer, including two vector vaccines, one inactivated vaccine based on a low pathogenic H5N2 virus variant, and a DNA vaccine. It was the vector vaccines which were found to significantly reduce the viral transmission and achieve a reproduction rate of less than 1 “The chickens [vaccinated with HVT-H5 vector vaccines] were completely protected against disease after infection with the HPAI H5N1 virus,” Minister of Agriculture Piet Adema said in a letter to the Dutch House of Representatives 15. This type of vector vaccine is only known to be effective in chickens and turkeys, and is administered at the hatchery in day-old chicks. Meanwhile, the other two vaccines were described as “less effective”. “They do provide some protection against disease symptoms and death, but insufficient [protection] against the spread of the H5N1 virus,” the letter asserted.

Why aren’t vaccines against HPAI more widely used in regions such as the US or Europe?

There are disadvantages to the existing vaccine technology which have to be considered when evaluating a vaccination program for HPAI.

Strain Matching

Like human influenza, HPAI is a moving target. “Over time, the viruses in the field that cause the disease and cause infection, they change. That’s part of being an influenza A virus. It’s an RNA virus. It doesn’t have proofreading in the replication cycle. So it changes, and so the vaccines have to change with it,” explained Dr. Swayne.

Dr. Swayne recalled observations from Indonesia in 2006-8, a few years after the country began vaccinating against HPAI. Chickens were vaccinated using a vaccine which, like the viruses circulating locally, had an H5 hemagglutinin protein, and then were challenged with different field viruses. As he explained, the vaccine in question offered “really good protection [against some strains]; no illness, no death, very little virus replication,” but then, faced with strains from other parts of the country, vaccinated chickens might experience 50% mortality, or even 100% mortality, after a challenge.

“[They were] fully vaccinated, they had antibodies, they just didn’t match the field virus…they didn’t have a perfect match. And so that allowed the field virus to replicate.”

Still, he pushed back against the characterisation that inactivated HPAI vaccines do not work well. “I think that’s one of the myths, that [inactivated vaccines] don’t work…those vaccines can work very, very effectively, as other vaccines can, but you have to use them correctly, and the vaccine seed string that’s used has to match the field virus antigenically.”