Makoto Kobayashi and colleagues (June 19, p 2129)1
raise a very important question about polio immunisation in the interim period between countrywide elimination of wild polioviruses and ultimate global eradication. They have shown that, in 1998, 4 years after the last wild poliovirus isolation from an indigenous polio case was made in China, the average routine oral polio vaccine (OPV) coverage with three doses in 5—9-month-old infants in De Hong prefecture in Yunnan province was only 41%. They argue that “maintaining high OPV coverage is essential until global polio eradication”, on account of “the ever present possibility of importation of wild poliovirus”. Should coverage be high in routine immunisation or in annual campaigns, or should it be high in both?
This question is relevant in several less-developed countries that have achieved elimination of wild polioviruses or are close to reaching this goal. I wish to highlight three issues.
First, in countries that were unsuccessful in eliminating wild viruses even with high routine three-dose OPV coverage, but were successful with national immunisation days (NIDs), or pulse immunisation,2
which tactic is more important to prevent transmission? In my opinion, NIDs are more important (even essential) than routine immunisation, which had failed. Field evidence is provided by the investigators themselves, because four imported polio cases did not spread infection in De Hong during 1995—96. Whether or not routine immuniation coverage is high, the widest possible coverage with NID doses must be ensured to interrupt or prevent transmission of indigenous or imported wild poliovirus, and to maintain it.
I must point out that the sample (5—9-month-old infants) selected by the investigators is biased towards low vaccine coverage. The interval between the two two-dose NIDs is 10 months. Babies born during this time constitute about 83% of all infants and about 17% of all children under 5 years. Their three-dose coverage does not reflect the overall coverage of children under 5 years. All of them become eligible in the 10th month for NID doses of OPV.1
Much of the deficiency of routine doses would be made up in the NIDs. Whereas routine doses do not interrupt wild virus transmission, NID doses do and also increase the individual's protection. A decision to discontinue pulse immunisation (national or regional) must be made only after careful assessment of the risk of importation.
Second, as more countries eliminate wild viruses and as the whole world progresses towards eradication, the likelihood of importation steadily declines. As in China, high-quality surveillance for acute flaccid paralysis and virological investigations are the key to detect importation, so that intervention to intercept transmission can be applied without delay. This is a lesson for every country, whatever the stage of wild virus elimination in that country.
Third, a decline in routine immunisation coverage of not only OPV, but also of other vaccines in the WHO Expanded Program of Immunisation (EPI) is one of the adverse effects of the high-profile and labour-intensive NIDs in some countries. This decline, in a way, is a price to be paid for prioritising polio for eradication and for choosing OPV to implement it, since in many settings NID campaigns are required to interrupt transmission because of low vaccine efficacy and poor herd effect of OPV.3
Resurgence of diphtheria, whooping cough, or measles should be expected in such situations. For this reason, high routine-immunisation coverage with all EPI vaccines must be maintained, less for the sake of polio and more for the sake of these other diseases.