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Images in Clinical Practice Indian Pediatrics 2000;37: 1023-1024 |
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Vaccine Potency: A Doctor’s Dilemma |
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OPV is the vaccine of choice for polio eradication. No wonder its inventor got a Nobel prize. It has always been a dilemma for the medical personnel to know whether the sample that they are administering to the child is potent or not, inspite of ideal stroage conditions. The present batch of live trivalent OPV manufactured on vero cell culture has potency indicator. The vaccine vial monitor (VVM) depicts small white square within large dark circle (potent). If the small square turns darker than the large circle, it indicates loss of potency. I have observed through various manoeuvres that the potency sign only denotes that the vial has been kept under required temperature but does not indicate the potency of the vaccine in it or presence of a fictitious pink fluid. Hence it may be important to check the potency and the authenticity of the vaccine periodically. The vaccine which was supplied earlier without VVM used to change its color on loss of potency. So the change of colour used to be a better and direct indicator of loss of potency. The partially used vaccine maintained at required temperature does not show loss of potency even after several days as indicated by VVM. It means that the present practice of discarding the vial after each vaccination session can be reviewed, so that the remaining vaccine can be reused in the next session, provided it is stored well to maintain "potent" sign. However, WHO recommends that each vial of vaccine should be discarded at the end of an immunization session. Recent research on the safety and potency of vaccine has led to a change in the policy on the use of opened vials of vaccine. This can save nearly 45 million dollars each year on purchase of vaccine(1). Ramesh Yelsangikar,
Dr. Yelsangikar has raised some important issues regarding OPV. While it is true that The World Health Organization has chosen OPV for poliomyelitis eradication, countries using the alternate vaccine, namely IPV, had already eliminated polio from their territories. Thus, either vaccine may be chosen for polio eradication. Some countries like Germany and the USA have lately switched from OPV to IPV to maintain eradication, in order to avoid the vaccine-associated paralytic polio (VAPP). When VAPP is also taken into consideration, then OPV eradicates only wild virus polio and only when the use of OPV is discontinued will all polio be prevented. As long as India continues to have wild poliovirus transmission, a few VAPP cases will have to be accepted as a small price to be paid for eliminating wild viruses, but the day we also eliminate wild viruses, the continued occurrence of VAPP will raise difficult problems, particularly from ethics point of view. On many occasions I have heard about the Nobel Prize for Dr. Albert Sabin, for developing OPV. He was never given the Nobel Prize. The first successful vaccine against polio was not OPV, but IPV, developed by Dr. Jonas Salk. Neither was he given the Nobel Prize. Other scientists like Dr. Cox and Dr. Koprowski had developed and field tested attenuated type 1 and type 2 poliovirus vaccines given orally, earlier than Dr. Sabin. However, Dr. Sabin’s success was in attenuating all 3 types of polioviruses and also developing a practical monkey spinal cord test for neuro-virulence of the vaccine viruses. Every batch of each type of vaccine viruses is tested in Rhesus monkeys and only after certification of lack of neurovirulence can the batch be released. For one batch of OPV it may take some 60 monkeys for neurovirulence testing, at the rate of 20 monkeys per type. This number includes the positive and negative laboratory controls also. The Vaccine Vial Monitor is not a marker of vaccine potency, as rightly pointed out by Dr. Yelsangikar. The VVM indicates the history of time-temperature exposure of the vial from the time of affixing it. If the vial of vaccine with VVM had been exposed to high temperature for a certain time, the color goes through a series of changes which have been calibrated to indicate the likelihood of the decline in vaccine potency. The actual potency of every batch of OPV released would have been tested at the Central Vaccine Testing Center at Kasauli prior to its release. The change of color of the OPV liquid itself is not an indication of decline in potency, but only a change in pH. With modern manufacturing technology, the pH is controlled without necessarily adding a color pH indicator. Therefore, we can totally ignore the color of OPV. Then, why do not other vaccine vials get the VVM label? As the WHO became gradually aware of unexpectedly low vaccine efficacy of OPV in developing countries, the first conclusion to which experts jumped was the hypothetical loss of potency due to heat inactivation. Hence, VVM was developed and mandated for OPV, but the original problem was not solved. The problem of low vaccine efficacy is now countered by repetitive doses for every child. Since VVM adds to the cost of the vaccine, and vaccine efficacy issues do not occur with other vaccines, there is no particular need for VVM on other vaccine vials. When the Expanded Program on Immu-nization was originally formulated, the stipulation was to discard at the end of the day, any vial which was partially used. Subsequently this rule was revised in order not to waste potent vaccine only to follow a rule, as reiterated by Dr. Yelsangikar. There is no need to discard partially used vials as long as they have not left the clinic premises and had been kept cool in between use. Only vials that had been taken out of the clinic are to be discarded at the end of the working day. This rule is for ample caution assuming that the vaccine might not have been kept cool between use. Incidentally, this change was according to my proposal in the WHO Technical Advisory Group meeting in Geneva. VVM was not yet available then. With VVM, the OPV with satisfactory color indication can and should be saved and used rather than wasted. That is the least we must gain for the extra cost of the VVM. T. Jacob John, |
