Situation: The public health nurse of a Rural Health Unit (RHU) holds a monthly immunization session at a Barangay Health Station (BHS) under the National Immunization Program (NIP). At 8:00 a.m., the nurse checks the vaccine refrigerator log: Day | 8:00 a.m. | 4:00 p.m. Monday | 5 °C | 6 °C Tuesday | 4 °C | 3 °C Today | -3 °C | - The vaccine vial monitors (VVMs) on the pentavalent vials show an inner square lighter than the outer circle. What should the nurse do FIRST with the pentavalent vials?

Two rules apply at once. The VVM shows only cumulative heat exposure, so a usable VVM says nothing about freezing. Pentavalent is an adsorbed, freeze-sensitive vaccine, and the -3 °C reading means it may have frozen, so the vials are isolated in the cold chain and shake-tested before any are used; vials that fail are discarded and the breach is reported.

Clinical situation analysis

The refrigerator log shows a reading of -3 °C at 8:00 a.m. today, which is below the recommended storage range of +2 °C to +8 °C for most vaccines. The pentavalent vaccine is an aluminium-adjuvanted, adsorbed, freeze-sensitive product. The vaccine vial monitor (VVM) on the vial shows an inner square lighter than the outer circle, which indicates that the cumulative heat exposure has not reached a level that would make the vaccine unusable. However, the VVM only reflects heat exposure over time and cannot detect freezing damage. A normal VVM reading therefore does not rule out the possibility that the vaccine has been frozen.

Watch out! A usable VVM does not mean the vaccine is safe after exposure to sub-zero temperatures. Freeze damage is a separate mechanism of potency loss that the VVM is not designed to capture.

Why freezing matters for pentavalent vaccine

Pentavalent vaccine contains aluminium salts as an adjuvant. When an adsorbed vaccine freezes, ice crystals disrupt the lattice structure that binds the antigen to the adjuvant. Upon thawing, the antigen and adjuvant can separate, forming larger aggregates or floccules. This reduces immunogenicity because the antigen is no longer presented to the immune system in the intended particulate form. The result is a vaccine that may look normal but has lost potency.

The literature review by Hanson et al. confirms that exposure to temperatures below the recommended range in the cold chain can decrease the potency of freeze-sensitive vaccines, leading to loss of vaccine investment and placing children at risk of vaccine-preventable illnesses ^[1]. The demonstration project in Tunisia similarly highlights that accidental freezing is a real risk for national immunization programs when freeze-sensitive vaccines are exposed to sub-zero temperatures ^[3].

Why the shake test is the correct first action

The shake test is a validated field method for detecting freeze damage in aluminium-based, adsorbed, freeze-sensitive vaccines. Kartoglu et al. validated the shake test against phase contrast microscopy as a gold standard using 475 vials of 8 different WHO-prequalified freeze-sensitive vaccines . The principle is that a properly suspended adsorbed vaccine appears uniformly cloudy after shaking, while a freeze-damaged vaccine shows rapid sedimentation with a clear supernatant and granular particles settling at the bottom.

In this scenario, the nurse has documented a sub-zero temperature reading and is holding pentavalent vials that are freeze-sensitive. The correct first step is to isolate the suspect vials in the cold chain and perform a shake test before any use. Vials that fail the shake test are discarded, and the temperature breach is reported through the appropriate cold chain incident reporting pathway.

Why the other options are incorrect

Option	Reason it is incorrect
1. Use them as usual, since their VVMs show no sign of damage	The VVM only measures cumulative heat exposure. It cannot detect freezing. Using vials that may have frozen risks administering a subpotent vaccine.
2. Discard them together with all other vials in the refrigerator	Not all vaccines are equally freeze-sensitive. Some vaccines, such as certain live attenuated vaccines, may tolerate freezing. Discarding everything without testing is wasteful and not protocol-based.
3. Use them today and report the low reading to the Rural Health Unit	Using a potentially freeze-damaged vaccine before testing places the recipient at risk of inadequate protection. Reporting is necessary but must not precede isolation and testing.

Cold chain and immunization session implications

The Korean multi-site cross-sectional study evaluated vaccine cold chain practices and found gaps in compliance with national guidelines and WHO recommendations on vaccine storage and handling . This underscores the importance of systematic temperature monitoring, proper documentation, and a clear protocol for responding to temperature excursions. In the context of a Rural Health Unit holding an outreach immunization session, the nurse must balance service delivery with vaccine safety. The priority is to protect the integrity of the vaccine before administering it to infants.

Key point! For any freeze-sensitive adsorbed vaccine with a documented sub-zero temperature reading, isolate the vials and perform a shake test before use, regardless of the VVM status. The VVM is a heat indicator, not a freeze indicator.