Dr Johannes Löwer, Germany

WHO guidelines on viral inactivation and removal procedures summarize current knowledge on virus inactivation and removal methods. They are intended to assist national control authorities and to provide guidance for manufacturers of blood products.

Human immunodeficiency virus (HIV), hepatitis B virus (HBV) and hepatitis C virus (HCV) are commonly tested plasma-borne viruses. Hepatitis A virus (HAV) and parvovirus B19 may be less of a problem in terms of contamination, although B19 can be dangerous in certain groups, such as pregnant women.

In studying viral inactivation, it is important to select the correct viruses for validation experiments. If possible, the virus of interest itself should be studied (this is possible for HIV and HAV, for example). Some viruses, however, such as HBV, cannot be cultured, and other viruses have to be used as a model. It is important to note that some model viruses do not properly reflect the behaviour of the relevant viruses on treatment. For example, porcine parvovirus (PPV) is not destroyed by pasteurization, but human parvovirus B19 is.

In modelling the production process, correct down-scaling is crucial. The robustness of virus inactivation or removal should be considered with respect to critical process parameters. An effective virus removal process should demonstrate more than 4 log₁₀ inactivation/removing capacity. Reduction factors of less than 1 log₁₀ cannot be considered.

There are various methods of inactivation.

• Pasteurization at 60°C for 10 hours. Critical factors affecting the process include temperature and concentration of stabilizer.

• Terminal dry heat (at least 80°C) or vapour heat (typically 60°C). Critical factors include temperature and strict control of residual moisture.

• Solvent detergent treatment. This does not affect nonenveloped viruses. Critical factors include the concentration of the reagents, avoidance of virus aggregates, and temperature.

• Incubation at pH 4 for between 6 hours and 21 days. Critical factors include pH and temperature.

• Cold ethanol fractionation. Critical factors include ethanol concentration, temperature, and filtration and centrifugation conditions.

• Chromatography. Critical factors include column load and height, chromatographic profiles, flow rates and conductivity.

• Nanofiltration. Critical factors include pressure, flow rate, filtration time, filter integrity and load, and composition of the intermediate product.

There is a danger that virus inactivation or removal methods may damage the proteins in the product. Therefore, the consistency and integrity of the product must be demonstrated.