The phrase 'major effects' is grammatically correct and can be used in written English.
For example, "Scientists are still studying the major effects of climate change on the environment."
It had two major effects.
What major effects might global warming have on this country's economy?
The committee's ultimate decisions could have major effects on consumers and taxpayers in the state.
There it will have two major effects, best envisaged as heat and acid.
US observers who follow Wal-Mart closely predicted major effects from its arrival in Britain.
Small, crucial savings in the energy of lifestyle has major effects.
The report also says the huge changes in education policies are evidence that mayoral control has had major effects.
The US shale gas boom has already had major effects on Russia, though its responses have remained ad hoc.
Mr. Pashley of the bird conservancy said that north of Edmonton, Alberta, oil and gas extraction had had major effects.
Across the country, the federal effort to flush out illegal immigrants is having major effects on workers and employers alike.
Though the number may seem trivial, even smaller differences in molecular maps can have major effects on viruses.
