Development and use of the H strain of avian infectious bronchitis virus from the Netherlands as a vaccine: a review

The H strain of infectious bronchitis (IB) was one of the earliest live attenuated IB vaccines to be developed and has continued to be used in most parts of the world for almost 50 years. It was developed for use at both the 52nd (H52) and 120th (H120) vaccine levels and, because of it ability to provide heterologous cross-protection against a number of IB viruses of different serotypes, has proved to be one of the most enduring live attenuated IB vaccines. In fact, the H120 vaccine is possibly the most widely used live attenuated IB vaccine globally to this day. The use of H52 has, however, declined with the introduction of safe and highly efficacious inactivated IB vaccines. This review documents the original studies to isolate and attenuate the H strain by serial embryo passage, and describes the early studies to demonstrate its efficacy in laboratory studies and under field conditions. The efficacy of the H vaccine in providing cross-protection against some of the many IB variants now reported worldwide is also discussed, and possible future vaccination strategies for IB considered.