Five US states were immune to the bloody, perpetual series of mass shootings in the United States this year, which has seen more of them than the number of days gone by.

Experts debate whether the states were spared thanks to coincidence or if circumstances there make them a haven of peace.

As of December 2, 353 mass shootings have killed 462 people in 220 cities, according to the shootingtracker.com website.

A total of 1,317 people were wounded, after adjusting for the latest toll from the last mass shooting, which saw a husband and wife couple kill 14 and wound 22 in San Bernardino, California, the deadliest such tragedy in three years.

If there is no slowdown to this frenetic pace, there will be as many such traumatic deadly events as there are days in the year. Or more.

The count includes all events that have killed or wounded at least four people.

Hawaii, New Hampshire, North Dakota, West Virginia and Wyoming alone were spared such macabre fate.

All of them except West Virginia have not seen a single mass shooting since 2013, when the website first began its count based not on official figures but on reports obtained from media reports and other sources.

The outcome owes in part to the relatively low population density in those states, experts say.

Wyoming, home to 584,000 people, is the least populous state, according to 2014 estimates from the US Census.

North Dakota, with 739,500 people, is the fourth least populated state (ranking 47th out of 50 overall by population).

"Naturally, we would expect that states with smaller populations would have fewer mass shootings, on average," University of Alabama criminologist Adam Lankford told AFP.

The most populous state, California with 38.8 million people, had the second biggest number of shootings so far this year -- 25.

Florida, which counted the most -- 27 -- shootings, has a population of 19.9 million, making it the third most populated state.

- 'Lucky this year' -

The five states are also among the most rural. Most lack major cities, except for Hawaii, with Honolulu having a population of about 375,000.

So people in these states are less likely to live in cities than those in most other US states.

"This affects their risk and probability of experiencing a mass shooting," Lankford said.

"Although school and workplace shootings do occur in towns and other rural areas, there are many types of mass shootings that mostly occur in cities, such as mass shootings that arise from gang violence, organized crime, and other criminal activity."

But their immunity to these incidents is counterintuitive.

None of these five states, except for Hawaii, has adopted strict gun control legislation, and it is often easier to own one there than elsewhere in the United States.

Wyoming namely does not regulate the transfer or possession of machine guns and no state permit is required to purchase a rifle, shotgun or handgun.

That earns it an F from the San Francisco-based Law Center to Prevent Gun Violence, an advocacy group. North Dakota and West Virginia also get that lowest possible grade.

New Hampshire fared just a hair better, with a D-.

Hawaii, in contrast, got a B+, because of its license and registration requirements, ban on assault weapons and large capacity ammunition magazines, child access prevention requirements and restriction on openly carrying of handguns and long guns.

An increasingly frustrated President Barack Obama has repeatedly called for stricter gun control legislation, but his calls have gone unheeded.

The easy flow of weapons, experts say, triggers more mass shootings because guns can end up in the wrong people's hands.

There have been numerous shootings in those five states, but always fewer than four victims.

"None of them have done anything innovative or effective to prevent mass shootings, it just happens to be an unfortunate coincidence," said Adam Winkler of the UCLA School of Law.

Winkler, who wrote "Gunfight: The Battle over the Right to Bear Arms in America," said there was "no doubt" that gun rights advocates will point to the lack of mass shootings in those states as proof that less gun control means more safety.

Yet "it is very difficult to reduce mass shootings, mainly because most of the guns used in these shootings are purchased legally," he added.

"I don't think it is possible to stop mass shootings at the state level, even if, of course, they can take some actions."

Lankford thinks it has more to do with coincidence than anything else.

"I think these states have been lucky this year, and in the future they are unfortunately more likely to experience some mass shootings," he said.

Mass shootings remain "very rare" events in the United States, with a population of nearly 320 million, said James Jacobs of New York University School of Law.

"It is unlikely that any policy initiative can prevent such rare events," he added, noting that California has some of the nation's strictest gun controls.

So shootings can take place at random, and "luck could continue" for those five states, according to Jacobs.