Recent Newcastle disease outbreaks in Spain have raised an important question for Europe’s poultry industry: if flocks are vaccinated, why can the virus still circulate?

The first outbreak in Valladolid was confirmed on 15 June 2026 at an unvaccinated broiler farm housing 24,000 birds. The situation then developed into a cluster of 14 outbreaks by 7 July. According to information presented by Spain to the European Commission, seven of the affected farms were vaccinated.

Laboratory sequencing identified the virulent field virus circulating in Valladolid as Genotype VII.2. This is an important finding because the Newcastle disease virus belongs to a single serotype, meaning existing vaccines can generally provide protection against clinical disease caused by different genotypes. However, belonging to one serotype does not mean that all viral genotypes are identical in their epidemiological behaviour or in the level of protection achieved against infection and shedding.

The Valladolid strain also differed from the Genotype VII.1.1 virus detected in the earlier Valencia outbreaks. Spanish authorities said this difference supported the conclusion that there was no epidemiological link between the two events, pointing instead to separate introductions.

Vaccination remains the most important tool for protecting birds against mortality and severe clinical disease. But clinical protection is only one part of an effective Newcastle programme. Conventional vaccines do not always provide sterile immunity: a vaccinated bird may survive exposure without major clinical signs while still becoming infected, replicating the field virus and shedding it into the environment.

This is where transmission control becomes critical. The objective should be not only to keep individual birds alive, but also to reduce the number of infected birds, the amount of virus they shed and the duration of shedding. Lowering these three parameters reduces environmental contamination and the probability of transmission within a poultry house, between houses and ultimately between farms.

Research indicates that vaccination can substantially reduce shedding compared with unvaccinated controls. It also suggests that vaccines more closely matched to circulating Genotype VII field strains may further reduce viral replication and shedding. Genotype matching does not necessarily determine whether a vaccine prevents death, but it may improve its capacity to reduce the epidemiological load and control transmission.

Field performance nevertheless depends on more than the vaccine strain. Application quality, timing, flock coverage, maternal antibodies, vaccine take and the uniformity of the immune response can all influence protection. A flock recorded as vaccinated is not automatically a uniformly immunized flock.

For producers, this makes Newcastle vaccination a transmission-control programme rather than a single farm procedure. Vaccination must be supported by immunity monitoring, early diagnostics and strict biosecurity governing the movement of birds, people, vehicles and equipment.

The Spanish outbreaks do not show that vaccination is ineffective. They show why vaccine performance must be evaluated through two complementary outcomes: protection against clinical and economic losses, and reduction of infection, shedding and transmission.