The basic Medicare premium will shoot up next year by 15%, to $110.50 a month, federal officials announced. The increase means that monthly premiums would top $100 for the first time, and about 27% of Medicare beneficiaries will have to pay higher premiums or have the additional amounts paid on their behalf. The other 73% will be shielded from the increase because, under federal law, their Medicare premiums cannot go up more than the increase in their Social Security benefits.