The volume of spam, dropping steadily since August, saw a precipitous decline around the Christmas period, they said.
During the Christmas holidays, three of the largest spam producers curtailed their activity, Paul Wood, a senior analyst at Symantec Hosted Solutions, told BBC News.
"But it's hard to say why," he added.
One security firm said some 200 billion spam messages were being sent each day in August but only 50 billion in December, the BBC reported Thursday.
Most spam is sent by networks of infected computers known as botnets, with one, known as Rustock, responsible at its peak for nearly 48 percent of all spam sent globally, Wood said.
In December, Rustock was responsible for just 0.5 percent of worldwide spam, he said.
One possible explanation is spammers are simply regrouping ahead of a new assault.
Spammers are driven entirely by profit, Carl Leonard, a researcher at security firm Websense, said.
"So if a campaign is not getting the returns they want, they can stop, regroup and try something else." he said.