Winter is often when infectious bronchitis virus (IBV) pressure rises — and when many poultry producers take a hard look at their live respiratory vaccination strategies.
In some regions, the threat of IBV flare-ups has shaped a long-held industry mindset: when bronchitis pressure rises, it may be safer to pull back on live Newcastle disease (ND) vaccination to avoid “stacking” respiratory vaccines.
“The concern is that combining live vaccines in a high-pressure season could intensify post-vaccination respiratory signs or affect how well birds respond to vaccination,” said Roy Jacob, DVM, PhD, senior key account veterinarian at Boehringer Ingelheim.
In practice, he added, that caution may push some programs toward alternative ND strategies during winter. However, new research suggests that the approach may deserve a second look.
In a controlled study, researchers at the University of California, Davis, evaluated a live VG/GA Newcastle vaccine used alongside a live (Mass) infectious bronchitis vaccine and found no negative interactions.
Furthermore, the study found that the combined program supported stronger memory response and mucosal immune activity when vaccines were applied properly, pointing to potential benefits beyond avoiding vaccine interference.
Why the VG/GA ND vaccine is different
To understand these findings, it helps to look at what makes the VG/GA strain distinct from other live Newcastle vaccines, Jacob said.
Not all live ND vaccines behave the same way in the bird. A key feature of the VG/GA vaccine used in the study is that it is primarily enterotropic, meaning it naturally replicates in the intestinal tract after administration, rather than in the upper respiratory tract.
That difference can matter when IBV vaccination is the key focus. If a live ND vaccine is less likely to drive strong replication in respiratory tissues, it may be easier to maintain live ND coverage during a period when the respiratory system is already under pressure. The milder respiratory component of the VG/GA strain stays in the upper respiratory tract, stimulating the immune organs and cells, which drives a stronger memory response if there is a second exposure.
Another interesting aspect of the VG/GA strain is its ability to stimulate immune responses beyond circulating antibodies alone, Jacob added.
“In previous work, we’ve seen that this strain can enhance immune responses, including macrophage activity and T-cell involvement,” he said. “So, in this study, we also wanted to understand what happens when you use it alongside bronchitis vaccination, and whether it enhances respiratory mucosal immunity.”
Study overview
The study was conducted with Rodrigo Gallardo, DVM, PhD, at the University of California, Davis, using specific pathogen-free (SPF) chickens.
Birds were vaccinated at day of age and assigned to groups that received either a live VG/GA strain Newcastle vaccine, a live IBV vaccine (Mass-type), or both live vaccines together.
Birds were monitored after vaccination, and sampling was performed to evaluate cellular and humoral immune response indicators. After a 14-day observation period, half of the birds in each group were challenged on day 15 with an IBV-1639 strain.
All birds were euthanized and necropsied at day 35. Throughout the study, researchers collected samples for histomorphometry and immune response evaluation, including IgA and IgY ELISA and flow cytometry of tracheal washes, to measure immune cell populations, such as macrophages, T cells and B cells.
No evidence of negative interaction
Results demonstrated that birds receiving both the live VG/GA strain ND vaccine and the live IBV vaccine did not show evidence of a harmful interaction compared to birds receiving either vaccine alone.
“What we saw in this work is that, when applied correctly, both live vaccines can be administered together without seeing the kind of negative interaction producers worry about,” Jacob said.
Immune response observations: Signals of stronger local activity
Beyond overall compatibility, the immune response patterns evaluated in the study may help explain why an enterotropic live ND option could be useful in a respiratory program. When birds received both vaccines, researchers observed indicators consistent with increased immune cell recruitment.
“Birds that received the VG/GA strain alone had increased tracheal macrophages after the DMV-1639 challenge, and the VG/GA + IBV Mass vaccinated birds demonstrated expanded T and B cell populations after challenge,” Jacob said. “There was significantly more immune cell recruitment when there was dual vaccination.”
The study also revealed elevated antibody signals in the dual vaccinated birds, including IgA and IgY.
“In particular, when you see higher IgA at the mucosal level, that’s pointing toward stronger local immune protection at the respiratory surface,” Jacob said. “And that local immunity is a big part of how birds handle respiratory challenges early on.”
Where vectored ND fits — and what it may not deliver
According to Jacob, those immune response differences are part of why producers still weigh live and vectored ND vaccines differently — especially when the goal is early, local protection in the respiratory tract.
Vectored ND vaccines remain valuable tools in many production systems, particularly when programs want strong ND protection with simplified logistics or minimal post-vaccine respiratory reaction. In many cases, they support consistent baseline immunity with less risk of vaccine “noise.”
However, Jacob noted, vectored ND strategies may not generate the same level of mucosal immune response that a live vaccine can provide.
“The vector vaccines — especially HVT-ND vaccines — don’t really produce strong mucosal immunity. This local immunity is also very important,” he said.
Furthermore, Jacob added, live vaccines tend to support earlier immune activation.
“A vector vaccine will give you systemic immunity, but a live vaccine tends to create earlier and more robust protection, which is especially important for early challenge,” he said.
No need to compromise ND protection
For producers, the message is less about adding complexity and more about avoiding unnecessary compromise, Jacob said. If a program needs strong bronchitis coverage during winter, the findings suggest that a live VG/GA ND vaccine can still be used alongside a live IBV vaccine without evidence of a negative interaction in SPF birds.
“There’s a dogma in the industry that when bronchitis pressure is high, you should step away from live Newcastle vaccination,” he said.
“But what we see in this study is that you don’t have to stop using a live Newcastle vaccine just because there’s a bronchitis challenge. You can use both together and gain enhanced mucosal protection.”
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