SÃO PAULO, Brazil, Feb. 15 (Reuters) — Merck & Company said on Friday that an unknown number of people in as many as 27 nations, including 60,000 youngsters in Brazil, might need new shots to prevent infection with the hepatitis A virus because vaccines they received might have been defective.

A unit of the French pharmaceutical group, Aventis, which sells the vaccine in Europe in a joint venture with Merck, disclosed in December that it was recalling batches of pre- filled syringes because they might not be potent enough to protect against the virus, which is spread by poor sanitation and can cause liver damage.

At the time, however, neither Merck nor the Aventis Pasteur unit indicated how many people who had taken possibly faulty batches made between December 1999 and December 2001 would need new vaccines.

Although the batches may have been ineffective in protecting against the virus, Merck said the vaccines were not harmful.

Gwen Fisher, a spokeswoman at Merck's headquarters in Whitehouse Station, N.J., said on Friday that the possibly ineffective shots of its hepatitis A vaccine given in the past two years include both the VAQTA K vaccine for children and the VAQTA vaccine for adults.

"Merck is offering to pay in most countries for either retesting people to see if they are effectively vaccinated or for revaccinations," said Ms. Fisher, who added that she did not know Merck's potential financial liability.

Merck commented on the potential number of people affected by the faulty vaccine in response to inquiries by Reuters following reports by local newspapers in Brazil about the use of the possibly ineffective vaccine in Brazil.

Merck sells the VAQTA K vaccine against hepatitis A for young people in the United States, Latin America, Asia and parts of Europe. The vaccine is put into syringes in Britain by Evans Vaccines, part of PowderJect Pharmaceuticals.

In Brazil, a spokesman for Merck's unit there, Merck Sharp & Dohme, said about 60,000 children and adolescents in the country could need another dose of the preventive drug.

Merck said the young Brazilians were among those who took about 117,000 possibly faulty vaccines in the past two years, mostly in private clinics in the industrialized states of São Paulo, Rio de Janeiro and Minas Gerais.

Each dose of the vaccine was given twice to the Brazilian youths and, although not harmful, it may not be potent enough to prevent the disease, the company said.

"The reaction varies from person to person and we cannot guarantee that the doses in the problem lots will be effective," said Marcos Levy, director of corporate affairs for Merck in Brazil.

Mr. Levy said oxygenated water had seeped past the seal in pre-filled syringes of the medicine packaged in Britain and also sent to the United States, France, Ireland and Germany.

"We recalled that lot and other lots that, although still valid, used the same type of seal on the syringes," Mr. Levy said.

He added that the Brazilian clinics which gave the vaccine are contacting patients so that they can return for testing and perhaps a new shot.

Unlike hepatitis B or C, hepatitis A is rarely deadly and only severe in about 2 percent of cases.