Superstorm Sandy is looming over the insurance industry as companies look to lock up their backup protection against large losses for the coming year.

The massive storm, which struck the most populous region of the U.S. in late October, helped moderate declines in U.S. property reinsurance rates, according to a new report from insurance broker Willis Group Holdings PLC (WSH). The price of such coverage was flat to down 5% for insurers that didn't have reinsurance losses in 2012, according to the report.

"In the absence of Superstorm Sandy, reinsurers would have found it difficult to resist buyer pressure for further concessions," wrote executives from Willis Re, the company's reinsurance arm. "Sandy's impact has helped to stabilize market pricing on an overall basis."

Rates were up 10% for insurers that did have reinsurance losses in the past year, the report said.

Rod Fox, chief executive of reinsurance broker TigerRisk Partners, said rates appeared to be headed down 5% to 10% before Sandy struck.

"Sandy slowed that decline," he said. While reinsurers have plenty of capacity to take on risk, "there's always a psychological impact on the market after a large catastrophe, where people say what if it had been even worse."

The start of January is one of the most important dates for reinsurers, when about half of all their business is done. Reinsurance contracts that renew in January include national accounts that cover large losses across the U.S., while June 1, another important renewal date, is focused on the Florida market.

The Willis report is about in line with recent predictions from analysts that track the industry. Reinsurers largely came through 2012 in excellent shape, as insured losses from catastrophes in 2012 were about half the $120 billion pricetag from 2011. Claims from Sandy, currently estimated at $20 billion or more, were dwarfed by the combined cost in 2011 of the Japan earthquake, U.S. tornadoes and flooding in Thailand.

Still, Mr. Fox said the storm gave insurers and reinsurers pause, in part because it caused significant damage despite being downgraded from a hurricane to a post-tropical storm before making landfall.

"You had a low wind-speed event that covered a massive area and had a huge flood component," said Mr. Fox. "A lot of people hadn't contemplated that kind of event. While not a complete shock, it was a unique set of circumstances."

Willis said one corner of the market was feeling a "disproportionate impact" from Sandy. The so-called marine market had its largest ever loss from Sandy, and also suffered from the sinking of the Costa Concordia cruise ship and the higher-than-expected cost of a container ship that ran aground off the New Zealand coast.

The losses caused "very late renewals," and many buyers of marine coverage are choosing to keep more of the risk on their own books to help offset rate increases of 15% or more, Willis said.

"Marine" insurance can involve coverage of ocean-going vessels, but the term is also used to describe policies on other types of transportation and the protection of goods while they are in transit.

Elsewhere in the world, Sandy had little impact on rates. For insurers that didn't draw on their reinsurance last year, property prices were flat to down 5% in Europe, flat to down 15% in China and flat to up 6% in Latin America, according to the Willis report.

Write to Erik Holm at erik.holm@dowjones.com

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