U.S. antiviral prescriptions spiked ninefold early this week after last weekend's media coverage about a possible flu pandemic, according to new data.

Prescriptions rose by ninefold on Monday and by nearly nine times on Tuesday compared to average daily volume in the rest of April, according to pharmaceutical market research firm SDI. The level marked the highest point so far this year for antiviral prescriptions.

The upward trend started on Friday, April 24, a day after the first public U.S. government announcement of cases of a new strain of flu, once known as swine flu, in California and Texas, SDI said. Prescription sales rose far higher in major metropolitan areas.

In Los Angeles, daily prescription volume jumped 16-fold on Monday to the highest level it has reached on any single day in the past two flu seasons.

In New York, volume climbed more than 14 times the daily April average on Monday, and rose to 15 times on Tuesday, according to SDI, which did not release data for later in the week.

"Although it is not uncommon for the number of antiviral prescriptions to increase at different times throughout the season, we don't expect to see such a dramatic increase in one day at the end of the season" said Laurel Edelman, SDI's vice president of clinical accounts.

The antivirals in the data include Roche Holding AG's Tamiflu and GlaxoSmithKline PLC's Relenza as well as several older drugs.

Shares of Roche and Glaxo jumped earlier this week as investors bet the threat of a flu pandemic would provide a windfall for the drugmakers.