Herd immunity can be effective way to prevent disease

In the last few months you may have heard experts on the COVID-19 pandemic discuss the concept of herd immunity as a way of slowing or stopping the pandemic.

It would seem that herd immunity is not an imminent possibility for the Canadian population with regard to COVID-19, but I thought it would be interesting to explore this concept to see whether it applies to our cattle herds.

One of the most important factors that affects the size of a disease outbreak when a virus or bacteria enters into a population such as a cattle herd is the proportion of susceptible individuals. For example, if a highly infectious virus such as infectious bovine rhinotracheitis virus (IBR) is introduced to a herd of cattle that has never been exposed to the virus and never been vaccinated, it will cause a very severe and serious outbreak of disease with a high proportion of the herd becoming infected.

The result in a herd of pregnant cows would be an “abortion storm,” in which a large proportion of cows abort due to the IBR virus infection. This is because the entire herd is susceptible to the virus and there is no immunity present in any of the individuals in the herd. This allows the virus to spread rapidly from individual to individual within the herd.

Epidemics of infectious disease are somewhat similar to a forest fire, and the proportion of susceptible animals in the herd is the fuel for the fire.

As the proportion of susceptible animals within the herd decreases (either through dying or developing immunity), the fire of the epidemic starts to slow and eventually sputters and dies.

If a significant proportion of the population or herd is immune to IBR virus because some of the cows have been vaccinated, this immunity actually provides protection to the cows that are not immune to the disease.

Cows that are infected with the virus come into contact with cows that are already immune and as a result are less likely to spread the virus through the herd.

By having a proportion of the herd vaccinated, we can actually help to lower the possibility of the virus being transmitted within the herd. What proportion of the herd or population needs to be vaccinated in order to help provide herd immunity? Well that depends on how contagious the disease is and a number of other factors. However, in most cases it would appear that having at least 70 to 90 percent of a population with immunity, such as a vaccine, will be enough to achieve herd immunity.

Measles was once a common infectious disease in children, but is now relatively rare in North America because vaccination rates have been high enough to establish herd immunity. In the last few years, there have been several outbreaks of measles in specific areas because of some anti-vaccination sentiments in some communities, thus reducing the level of “herd immunity.”

One of the places that we have seen herd immunity at work is in some of our clinical trials of vaccines.

Most of our vaccine trials involve taking a herd or pen of animals and randomizing the individual animals to either receive the vaccine or a placebo. We then evaluate who gets sick to determine if the vaccine is effective.

However, the mere act of vaccinating half of the animals in the pen can create some level of herd immunity, which will decrease the effects of an infection on the unvaccinated animals in the pen. This will decrease the potential differences between the vaccinated animals and the animals that receive the placebo injection simply as a result of herd immunity.

To avoid this, the clinical trial researcher has to assign the vaccine to pens of animals rather than to individual animals. This requires a much larger trial and is more expensive to carry out, but it often can demonstrate vaccine efficacy that we couldn’t identify when all of the animals are commingled in one pen or herd.

Does herd immunity mean that we should only vaccinate 80 percent of the cows in our herd? Definitely not.

Vaccines are a key component of our herd health programs and they are usually one of the most cost-effective disease control strategies that we have available to us. It would be foolish to leave any of your animals unvaccinated even though herd immunity may help to protect them.

However, herd immunity may be helpful when our vaccines aren’t perfect. Very few vaccines provide 100 percent protection and as a result, herd immunity becomes important in helping to reduce the spread of infectious disease even if the vaccine doesn’t work in every individual animal.

Vaccine efficacy can also be diminished by anything that negatively impacts the immune system such as inadequate nutrition or poor environmental management.

Work with your veterinarian to design a vaccination protocol that is appropriate for your herd.

Exposure to other herds, geographic differences and other risk factors makes every herd unique and as a result, vaccination protocols may differ from herd to herd.

Having a herd that is completely vaccinated with the appropriate vaccines gives you the best level of herd immunity.

John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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