Indonesia's nationwide drive last week to vaccinate about 24 million children against a spreading polio outbreak achieved most of its goal, but some parents continued to resist because of safety concerns, health officials said Monday.

The Indonesian Health Ministry reported that the campaign had reached 90 percent of children younger than 5 years old, despite lingering concerns among parents and medical workers about the safety of the vaccine.

"It was good but not great," said David Hipgrave, UNICEF's chief for health and nutrition in Indonesia, adding that the measure of success was uneven across the country. "The message is not getting out as well as it should."

After Indonesia carried out a regional immunization drive earlier this year on the western end of Java island, local news media reported that the vaccine had sickened several children, killing four. Health officials stressed that the vaccine was safe, even for ill children, but many parents vowed to keep their children from participating in the nationwide drive last Tuesday.

Most parents who refused to participate last week claimed their children were sick and thus unfit to be immunized. Ignoring the advice of international health experts, even some doctors declined to vaccinate children because they were unwell, U.N. officials said.

Indonesia is now planning another nationwide drive in three weeks to provide booster vaccines and to immunize children who were missed last week, health officials said.

Indonesia had been free of polio for a decade until a traveler from the Middle East carried the virus to Java early this year. Since March, Indonesia has recorded the highest rate of new cases in the world. The disease has infected at least 230 people, spreading from the mountains of western Java to the adjacent island of Sumatra and the capital, Jakarta.

According to Health Ministry figures, the results of the vaccination drive were disappointing in Banten, one of the hardest-hit provinces, where more than a fifth of the children under 5 were not immunized. Other provinces with outbreaks, including Jakarta, fared closer to the national average.

The campaign confronted not only widespread rumors about safety but the mammoth challenge of delivering the vaccine across the sprawling archipelago. Though thousands of health workers were dispatched, many by boat and aircraft, to reach remote settlements, many children were missed in these areas. For instance, on Irian Jaya, the impoverished, mountainous island at the far east of Indonesia, fewer than half the children were immunized, according to Health Ministry figures.

As the outbreak spreads in Indonesia, U.N. health officials have started warning that the virus could jump borders, infecting people in other countries in a region that had been entirely free of polio.

On Monday, state-run media in Vietnam reported that 2.3 million children in that country would be immunized against polio as a precaution. The effort would target children younger than 5 living in border provinces and other villages deemed to be at high risk.

The Philippines also announced this month that it would vaccinate 600,000 children living on the southern islands near Indonesia.

An Indonesian child receives a polio vaccination in Jakarta. A drive to immunize 24 million children last week was hindered by fears over safety.