The Greater Boston office market has swung into recovery mode, with plummeting vacancy rates in Back Bay towers and Route 128 office parks alike.

But out along the western portions of Interstate 495, where Fidelity and Hewlett Packard are shedding corporate campuses and EMC is trimming real estate as well, it looks like 2009 all over again.

Millions of square feet of empty corporate space are hitting the market along this key stretch of I-495 from Marlborough to Milford, with no end to the bleeding in sight.

In some ways, what’s happening along 495 West is a microcosm of the Bay State’s economy right now, with boom-time like growth in some sectors offset by ruthless cost cutting and downsizing in others.

“Those three blocks alone – that is a lot of space,” noted Brendan Carroll, research director at Richards Barry Joyce & Partners, of EMC, Fidelity and Hewlett Packard and the recent frenzy of corporate real estate cuts on 495.

Three Strike

As office vacancy rates come down across Greater Boston, along 495 West, they are poised to spike.

The vacancy rate in the Back Bay, for example, is now headed below 10 percent into the single digits, while along Route 128, along the densely packed Burlington to Dedham stretch, it has dipped below 20 percent.

But along 495 West, the vacancy rate currently stands at more than 29 percent, according to Colliers International. And while the second quarter numbers have yet to come out, the battered 495 submarket, with its once packed corporate campuses, could see the amount of empty office space soar past the 30 percent mark.

And those three big corporate real estate retrenchments are a big culprit in what has left the 495 West market reeling.

First, Fidelity Investments is shutting down its Marlborough campus and moving hundreds of workers out of state, putting more than 700,000 square feet of empty office space back on the market.

Second, Hewlett Packard is selling its 109-acre, 740,000-square-foot campus in Marlborough just off 495, having already moved its workers out at the end of last year.

And third, EMC is preparing to relocate a 300,000-square-foot data center from Westborough to North Carolina.

That’s more than 1.7 million of square feet of office space along the 495 corridor that is suddenly in play.

Eggs In One Basket

So why does it still look like the Great Recession along 495 while barely 17 miles to the east it looks like boom times again along 128?

Well part of it has to do with what’s hot and what’s not in the current economy.

Right now, life sciences companies and software giants are hot. And both sectors, having outgrown Cambridge’s increasingly crowded lab and office markets, have made the jump to 128, which is just close enough to the Mother Ship of innovation around Kendall Square to work.

By contrast, the 495 West market is dominated by the big companies that, even as they post big profits again, are still caught up in cost-cutting mode.

Fidelity’s decision to ditch its Marlborough campus is part of a long-standing drive to shift back-office operations to lower cost states. Hewlett Packard has moved workers to New Hampshire, while EMC’s decision to relocate a data center to North Carolina speaks for itself.

Nor is it clear where the next wave of growth will come to fill these corporate ghost towns. Eventually 128 will overheat, sending big life sciences or tech tenants in search of bigger quarters at lower rents.

But given the up and down nature of this recovery, that is hardly going to happen overnight.

If nothing else, the woes of 495 West are likely to strip some of the shine off the allure of the once-illustrious corporate campus.

Back when the economy was booming, the decision by corporate behemoths to invest in sprawling campuses seemed like an unalloyed boost for the 495 West market, and for Marlborough, Hopkinton and other communities that would be reaping the tax revenue.

But the recent cuts now highlight the downside to this strategy, which is akin to putting all your eggs in one basket. While buildings with lots of smaller companies may lack the glamour of the big corporate spread, there is an inherent stability in numbers.

So for a market like 495 West dominated by a few big corporate giants, all it takes is one or two downsizing decisions to send the vacancy rate soaring. “There are fewer multi-tenant buildings,” noted Mary Sullivan Kelly, senior vice president and chief research officer at Colliers International, of the 495 West market. “When one company makes a decision, it hits in a bigger way."