VVM type - how is it determined?

In your example, even though both Td vaccines contain the same tetanus and dipheria toxoids, it is likely that small differences in stabilizers, buffering systems, and perhaps differences in the adjuvants and or manufacting & packaing materials. Manufacturers try to improve formulation during the development stage and can possibly make small tweaks with the formulation post licensure (this is tricky), to improve stability. Again in your example of 2 specific Td vaccines, note that there is a difference in antigen concentration of T between the 2 vaccines (in I.U.). 

More generally the factors across all vaccines effecting heat stability of a vaccine are:

1. Nature of the Antigen

• Live attenuated vaccines (e.g., OPV, measles) are highly sensitive to heat because live viruses degrade quickly at elevated temperatures.
• Inactivated vaccines (e.g., Hepatitis B) and subunit or recombinant vaccines are generally more heat-stable because they do not contain live organisms.
• mRNA and DNA-based vaccines tend to be unstable and require ultra-low temperatures unless specially formulated.

2. Formulation Type

• Liquid vaccines are more prone to heat degradation than lyophilized (freeze-dried) vaccines, which are more stable in their dry form but become heat-sensitive after reconstitution.
• The presence of stabilizers, buffers, and excipients can improve heat tolerance by protecting the antigen structure.

3. Presence of Adjuvants

• Aluminum-based adjuvants (common in many inactivated vaccines) can be sensitive to heat, leading to precipitation or reduced immunogenicity.
• Some novel adjuvants (e.g., lipid nanoparticles in mRNA vaccines) are also heat-labile.

4. Moisture Content

• Higher residual moisture in lyophilized vaccines reduces stability. Proper drying during freeze-drying enhances heat tolerance.

5. pH and Buffer System

• Antigen degradation is accelerated outside optimal pH ranges. A well-buffered formulation helps maintain stability under heat stress.

6. Packaging and Container Type

• Glass vials provide better thermal protection compared to plastic.
• Single-dose vs. multi-dose vials: Smaller vials warm up faster, affecting stability during transport and handling.

7. Concentration of Vaccine Antigen

• Over-concentration or under-concentration can make vaccines less stable, affecting their resistance to thermal stress.

8. Stabilizers and Excipients

• Sugars (e.g., trehalose, sucrose) and amino acids can protect antigens by forming a "glass matrix" in lyophilized vaccines, improving heat stability.
• Lack of adequate stabilizers leads to rapid potency loss.

Hope this helps. regards JP