Summer officially arrives tomorrow, but vacations this year might not be the same anymore, if at all.

About half of US workers are skipping some, or even all, of their vacations days, and sticking to the grindstone for fear of losing their jobs while away on holidays, says new research.

“Employees are saying, ‘If I take off, it might show that I’m no longer necessary or relevant,’ ” said analyst Libby Bierman of Sageworks, a financial research firm that tracks private companies here and aboard.

It said one study showed Americans have an average 18 vacation days a year, but 48 percent forfeited as much as half or more of their days off in 2011, citing heavy workloads or fear of job loss.

Private firms posted a sharp jump in profit-per-employee to a current $14,985 per employee from $10,707 at the recession’s bottom in 2009, said Sageworks.

That study also said 63 percent of employees bring along family members on a business trip.