Avian influenza: three vaccination scenarios for poultry

A crisis on an unprecedented scale

Like many other countries in the world, France has experienced several major outbreaks of highly pathogenic avian influenza (HPAI) in recent years. The one in 2021-2022 was particularly severe, leading to the culling of more than 21 million poultry and endangering the French avian gene pool. Moreover, endemicity (persistence of the virus throughout the year) is now strongly suspected in wild birds in many parts of the country.

Poultry vaccination should be seen as a measure intended to supplement the preventive biosecurity measures taken in the field, such as the confinement of animals, the limitation of bird transfers, the disinfection or changing of clothing and equipment when moving from one farm to another, etc. To date, only one vaccine for chickens (Gallus gallus species) has a marketing authorisation in France. Applications for marketing authorisation or temporary authorisation for use have recently been submitted for vaccines for several poultry species; they are currently being assessed by the French Agency for Veterinary Medicinal Products (ANMV) and the European Medicines Agency (EMA).

Adopting a preventive vaccination strategy

An emergency vaccination strategy is not considered scientifically appropriate. The Agency recommends that the vaccination strategy implemented in autumn 2023 should be preventive and should have several objectives:

• anticipate epizootic outbreaks in order to avoid the massive spread of HPAI from infected farms;
• protect poultry sectors by preserving the gene pool in the country and consequently the ability to restart production after a possible animal outbreak.

In general, vaccinating the most exposed poultry will also limit the spread of the virus and the risk of mutation to prevent it from adapting to mammals and humans.

“We have not chosen the solution of emergency vaccination because of the long period between the vaccination of an animal and its protection against the virus; it takes an estimated three to four weeks for this immunity to be acquired. Furthermore, vaccinating animals in the middle of an animal epidemic tends to increase the flow of people on farms and therefore the risk of biosecurity breaches and also the risk of HPAI being introduced onto these farms” explains Caroline Boudergue, Deputy Head of ANSES’s Unit for the assessment of risks associated with animal health, welfare and nutrition, and vectors.

Determining which types of farms and which poultry species should be vaccinated as a priority

To develop this strategy, three scenarios are being proposed with regard to the risk of introduction and spread of HPAI. These scenarios have been established according to a graduated approach, based on the means available for vaccination:

• Scenario 1: Vaccinate animals present on nucleus and multiplier farms for all sectors (see box). This step has the advantage of requiring a limited number of doses of a vaccine. It can also protect French poultry sectors from the impact of a new animal epidemic by preserving genetic potential and the ability to put animals back onto production farms once the epidemic is over;
• Scenario 2: On production farms, vaccinate ready-for-gavage water fowl (ducks and geese), other free-range water fowl, free-range turkeys, and pre-adult layers (pullets) intended for free-range farming. The objective is to limit the scale of outbreaks by targeting the production sectors where the virus is most likely to be introduced and spread. This scenario requires that vaccines be more widely available than in Scenario 1;
• Scenario 3: If enough vaccines are available, vaccinate water fowl raised for meat production, turkeys raised in confinement (in buildings), free-range broiler Galliformes other than those listed for Scenario 2, and free-range laying hens.

Four necessary conditions to ensure the effectiveness of these scenarios

The effectiveness of the proposed scenarios will depend on the following assumptions and conditions:

• the epidemiological context in autumn 2023 will be the same as that observed today (H5N1 viral strain similar to the one that circulated during the 2022-2023 season);
• the available supply of vaccines will allow each species to be vaccinated and will induce collective immunity;
• the vaccination strategy will be consistent with the time required to acquire immunity after administration of the vaccine and with the duration of protection provided by the vaccine;
• the vaccination strategy will be compatible with farming practices, particularly from a logistical and economic point of view.