Two people were killed on Monday evening when a Probox rammed into a vehicle which was part of UDA Bonchari candidate Teresa Oyioka's convoy.
The two who were in the Probox - a man and woman-died as they were being rushed to Kisii Teaching and Referral Hospital.
Five other people, among them supporters of Oyioka, are undergoing treatment at the hospital.
A doctor said they are in stable condition.
The Probox was said to be overtaking the convoy which was on the campaign trail, when it rammed into one of the vehicles carrying supporters of the United Democratic Alliance candidate.
Oyioka was among the eight candidates cleared by IEBC on Monday to run for the Bonchari seat.
Oyioka is the widow to the late area MP Oroo Oyioka.
She was in a separate car and was not injured, said South Mugirango MP Sylivanus Osoro who was among those in the campaign team.
Police in Kisii have launched investigations.
Earlier, IEBC cleared Pavel Oimeke to run for the seat on the Orange Democratic Movement ticket.
The by-election has attracted at least 13 candidates.
Others who were cleared on Monday are Kevin Mosomi (Party of Democratic Unity) Jona Onkendi (The New Democrats) and Victor Omanwa (Party of Economic Development).
Others are Charles Ndege (Progressive Party of Kenya), Atancha Jeremiah (Agano Party) and Paul Mogiti ((Mwangaza Party).
At least five candidates, among them Jubilee's Zebedeo Opore, former Woman Rep Mary Otara (United Green Movement), Mary Nyabuto (Maendeleo Chap Chap) and Nyakeiyo Oigo (National Reconstruction Alliance) will be submitting their papers on Tuesday.
