A frozen vial looks entirely normal once thawed. The CDC's Vaccine Storage and Handling Toolkit (July 2026) states it plainly: inactivated vaccines may not appear frozen even when they have been exposed to freezing temperatures. The decision is therefore made with instruments, and each instrument measures something different.
| Tool | What it shows | Basis |
|---|---|---|
| Freeze indicator | That the temperature went below -0.5 °C, irreversibly | WHO PQS E006/IN07.1, clause 4.2.3 |
| Vial monitor (VVM) | Accumulated heat exposure; it does not measure freezing | WHO, VVM page |
| Data logger | The full temperature profile over time | CDC Toolkit, 2026 |
Under the WHO specification, a freeze indicator must reach its endpoint within 60 minutes at a temperature colder than -0.5 °C. The trip window is also defined: 98% of indicators must trigger between +0.5 °C and -1.5 °C.
A tripped indicator does not mean that freezing happened. In the validation study published in the Bulletin of the WHO in 2010, 117 vials were held at -2 °C for 24 hours; none froze, and all stayed liquid. In the words of the paper, freeze indicators are the only practical tool for documenting exposure to negative temperatures.
How the shake test is done
The test is defined only for aluminium-adsorbed vaccines: diphtheria, pertussis, tetanus, hepatitis B and Hib vaccines. Two vials from the same batch are taken. One is deliberately frozen and thawed and serves as the control. Both are shaken together by hand and placed side by side. If the suspect vial settles as fast as the control or faster, it has frozen; if it settles clearly slower, it has not.
| Experiment | Result |
|---|---|
| 475 vials, 8 vaccine types, 10 manufacturers | Shake test and phase-contrast microscopy agreed completely: sensitivity, specificity and positive predictive value of 100% |
| 156 frozen, 319 non-frozen vials | The test separated both groups correctly |
| 117 vials, -2 °C, 24 hours | None froze |
In the study, health-care workers with no previous experience of the test learned it from a standard learning guide with demonstration and coaching, and read the results with 100% accuracy. The method can be learned, but the test does not state how long or how cold the vial was. It does not replace the record.
What to do when there is doubt. The responsible person or authority holder makes the decision. Where there is doubt, the sequence is:
- The vial is set aside without use and labelled 'possibly frozen'.
- The data logger record and the freeze indicator status are written down.
- The marketing authorisation holder or the responsible person is informed.
- No stock movement is made until the written decision arrives.
The logger and where it sits. The logger probe belongs in the centre of the unit, surrounded by the vaccines. A device placed near the walls, floor, ceiling, door or an air vent may not represent the temperature of the product (CDC Toolkit, section 3). In household refrigerators, the CDC advises that vaccines are not kept in the freezer compartment and not on the top shelf under the cooling vent: air from the freezer can expose refrigerated vaccines to freezing temperatures.
In transport, one of the sources of freezing is a coolant pack placed in the same box as the vaccine. The CDC advises that frozen gel packs or coolant packs from an original vaccine shipment are not used in a cold chain box; even when they look sweaty, they can still freeze vaccines.