Wall Street believes in Amgen again.

Shares of the Thousand Oaks-based firm, the world’s largest biotechnology company, jumped 16 percent between market close July 25 and July 28, trading at volumes nearly five times higher than average.

With 7,500 employees at its headquarters, Amgen is Ventura County’s largest employer and the Tri-Counties’ largest publicly traded company.

“For Ventura County, it’s huge,” Bill Watkins, executive director of the University of California Economic Forecast Project, said of Amgen’s recent rally. When Amgen is doing well and handing out bonuses, Watkins said, Ventura County’s average salary goes up.

The major boost to Amgen’s stock price – which added nearly $8 billion to the company’s market capitalization – came in late trading July 28, after the company announced promising clinical results for its osteoporosis drug denosumab, or D-mab. Analysts say D-mab is likely the company’s next blockbuster drug.

Amgen’s shares fell slightly throughout the trading July 28 but crept back up after a positive earnings call after market close. Profits beat expectations and the company raised its 2008 earnings forecast.

Sales of Amgen’s top drug, Aranesp, which has been beleaguered by safety concerns, took a softer-than-expected blow; company officials said its declining sales have stabilized. Revenue was up across the rest of the company’s portfolio.

“Our overall financial performance is stronger than expected,” chief executive officer Kevin Sharer told investors on the call.

Amgen’s July 28 performance bucked national trends, with the company’s stock rising as the Dow Jones Industrial Average fell more than 240 points. Amgen was the top gainer in the Standard & Poor’s 500 index, one of only nine stocks that rose in the index.

Before its second quarter earnings call, Amgen had previously issued an upper revenue guidance of $14.6 billion, but raised that to $14.9 billion. It updated its best adjusted earnings per share estimate from $4.30 to $4.45.

Profit excluding certain costs clocked in at was $1.14, beating the $1.03 average estimate of analysts surveyed by Bloomberg News.

Amgen’s stock closed at $53.92 on Friday, hitting a peak of $62.50 in trading late Friday. The stock closed at $62.28 on July 29.

Amgen cut nearly 1,500 jobs last year. The D-mab developments are positive, but it’s not clear whether they will translate into a boon for the company’s headquarters, which handles mostly research and administration.

“It’s been our understanding that the reductions that were made last fall were it for some time,” said Gary Wartik, economic development director for the city of Thousand Oaks. “[D-mab] may bring back some employees – we don’t know.”

On the earnings call, Sharer tackled questions from analysts on whether Amgen would launch D-mab itself or partner with another company.

Sharer said the company would “take the kind of high science in medicine approach that we’ve traditionally taken. We do not see [D-mab] as the kind of normal, everyday sales item that a physician can just throw in the bag.”

Sharer answered sharply a Morgan Stanley analyst’s question on whether Amgen would enter the primary care space to sell its treatments. “That’s an unusual thought,” Sharer said. “I can’t imagine buying a company to get a sales force. That’s inconceivable.”

Although Amgen’s recent rally has been welcome news in Ventura County, challenges remain. November elections could bring an altered regulatory environment and changes in government reimbursements for drugs – a major source of revenue for Amgen.

“If they take a big hit, then you revert back to the worst of all possible situations, where we’re still dealing with Countrywide then trouble at Amgen,” Watkins said. “It would be terrible for Thousand Oaks in particular.”