The survival time of viruses in a virus transport medium (VTM) kit is affected by various factors, as follows:

I. Characteristics of the Virus Itself

1. Virus Type
   • Different types of viruses vary significantly in their survival time in VTM. For example, some enveloped viruses (such as influenza virus and SARS – CoV – 2) are relatively fragile and can generally survive for several days in VTM. The influenza virus can usually survive for about 2 – 7 days under suitable VTM conditions. SARS – CoV – 2 can survive for about 7 days at around 4°C in VTM. In contrast, non – enveloped viruses (such as adenovirus and norovirus) have a relatively stable structure and may survive for a longer time. Norovirus can survive for several weeks in VTM.
2. Virus Resistance
   • The resistance of a virus to environmental factors also affects its survival time in VTM. Viruses with strong resistance can survive longer in the medium. For instance, the poliovirus has a relatively high tolerance to chemical substances and physical factors. Under appropriate conditions in VTM, it can survive for a long time, perhaps several weeks or even months. This is because its nucleic acid structure is relatively stable and can withstand certain temperature, pH changes, and other external factors.

II. Components of the Medium

1. Nutrients
   • The nutrients in VTM are crucial for the survival of viruses. Adequate nutrients such as amino acids and sugars can provide the energy and material basis required for the virus to maintain its structure and activity, helping to extend the virus’s survival time. For example, some complex virus transport media are supplemented with a variety of essential amino acids and specific sugars, enabling viruses (such as certain enveloped viruses that require specific nutrients to maintain the integrity of their envelopes) to survive longer in them, potentially extending the survival time by 1 – 2 days compared to ordinary media.
2. Protective Agents and Stabilizers
   • Protective agents (such as bovine serum albumin) and stabilizers (such as certain polysaccharides) in the medium can prevent virus inactivation. Bovine serum albumin can form a protective film around virus particles, reducing physical and chemical damage to the virus. For example, in a VTM containing an appropriate concentration of bovine serum albumin, the survival time of some environmentally sensitive viruses can be significantly extended, perhaps from a few hours to several days. Stabilizers can maintain the structural stability of the virus, preventing its aggregation or decomposition, thus prolonging the virus’s survival time.

III. Storage Conditions

1. Temperature
   • Temperature is one of the key factors affecting virus survival. Generally, lower temperatures are beneficial for virus preservation. At a refrigerated temperature of around 4°C, the survival time of most viruses in VTM is extended. For example, storing a VTM containing virus samples at 4°C can potentially double or triple the virus’s survival time compared to storage at room temperature. At cryogenic temperatures of – 20°C or – 80°C, viruses can survive for several months or even longer. However, repeated freeze – thaw cycles may damage the virus structure and reduce its viability.
2. pH and Osmotic Pressure
   • Maintaining a stable pH and osmotic pressure in VTM is also important for virus survival. An appropriate pH value (such as 7.0 – 8.0) can prevent virus protein denaturation and nucleic acid degradation. If the pH value is outside the virus – suitable range, the virus survival time will be greatly shortened. Similarly, an appropriate osmotic pressure can ensure the integrity of the virus’s morphology and structure. When the osmotic pressure is unbalanced, the virus may burst or shrink, leading to its inactivation and thus shortening its survival time in VTM.