Vaccine quality is key to limit animal disease outbreaks

Vaccine quality control

Good vaccines don't always have to match the current strain of the disease exactly. If an FMD vaccine is of high quality, it can still provide solid protection against variants within the same serotype, according to researcher in foot-and-mouth disease (FMD), swine vesicular disease, and vesicular stomatitis at Wageningen Bioveterinary Research (WBVR). For example, during a recent foot-and-mouth disease outbreak in Uruguay, a vaccine containing an older vaccine strain helped reduce the number of cases before the second round of vaccination was completed. Centralised FMD vaccine quality control was the basis for FMD control in South America, illustrates the WBVR researcher.

Each dose of standard vaccine should contain at least 3 PD₅₀, which is three times the amount needed to protect 50% of the vaccinated animals. This level typically results in about 75% protection across a herd - enough to reduce the risk of disease spread. “But for outbreaks with variant viruses a higher PD₅₀/dose is required.”

Lab tests versus real life

Measuring long-term protection in a population isn’t easy. Right after vaccination, animals often show strong protection, even though antibody levels (titers) might not have reached their maximum level yet. Months later, the same levels may not be enough for protection against the so-called needle challenge. According to Dekker, this shows that lab tests often don't match what happens in real-life situations. “In the field, animals don’t get a needle in their tongue,” Dekker says, referring to the fact that some lab tests aren’t realistic. “Test such as the needle challenge should only be used shortly after vaccination”, he advices.

Dekker recommends that farmers and veterinarians run small tests on 5 to 10 young animals that haven't been vaccinated or exposed to disease before. By testing them 21 days after vaccination, they can see how well the vaccine is working. Right now, that's not done often enough, in the opinion of Dekker.

How to test efficacy

There are two main methods to test vaccines: the virus neutralisation test (VNT) and ELISA, the researcher says. VNT gives better predictions about how well a vaccine protects, but there can be a lot of variation between tests. ELISA is easier to standardise and more consistent, and has been used successfully in some regions, such as South America.

To measure cross-protection for understanding how well a vaccine works against different strains of a disease, scientists use the r₁ value. If this value is too low, the vaccine may not protect well against new strains. Still, the researcher is confident that well-made vaccines can protect against many different strains, even the older vaccine strains.

Using vaccines with more than one strain can also be a very useful approach, according to Dekker. “But sometimes the addition of a second strain can reduce the immune response to the first strain, so this needs to be managed carefully.”