JERUSALEM (JTA) — A nationwide campaign has begun to inoculate all Israeli children ages nine and under with weakened, live form of the polio virus.

The decision comes in response to the discovery in May of the polio virus in waste water in Israel’s South that reportedly had been there since February. The virus was found about a month ago in waste water in central Israel.

More than 50,000 Israeli children living in southern Israel have been inoculated with the live virus in the last two weeks. The three-month campaign now is being spread to central and northern Israel.

The children already have been inoculated against polio in their regular childhood vaccinations.

The purpose of the extra vaccine is to pass the weakened virus to adults with whom the children come into contact who may not previously have been vaccinated.

Across Israel, the vaccination rate against polio is 94 percent, according to the World Health Organization, which is supporting the vaccination campaign and whose representatives reportedly called it “necessary.”

After being detected in May, the virus was found in samples from at least 10 sites in Israel, mostly from the southern part of the country, according to WHO. In July, the virus also was found in sewage systems in Ramle, Lod, Modi’in and communities in the center of the country.

It is believed the virus was brought to Israel from Egypt; polio was discovered in sewage in Egypt in December. The same virus also is prevalent in Pakistan.

Israel experienced its last case of polio in 1988.