She's called on other mums-to-be with a public profile to help persuade women it's safe to get the vaccine. Research released last week found that catching Covid can be devastating during pregnancy.
And the 33-year-old says any sympathy she had with anti-vaxxers evaporated when protesters hijacked a 9/11 commemoration at Belfast City Hall last September.
Kate, the Alliance councillor for Balmoral, has won praise from the Royal College of Midwives for speaking out about her decision to get her booster, in response to the Department of Health's Get Boosted Now campaign aimed at pregnant women.
She's 23 weeks pregnant and had her third injection on December 17.
"I have had my booster and I've since had my 20-week scan and everything is fine.
"Hopefully that would encourage some women who are reticent about the vaccine to talk to their midwives or GP," says Kate.
"It wasn't difficult for me because I know that I'm not an epidemiologist or a medical professional and I'm used to putting my trust in people who are."
The first citizen says she can appreciate that some women are unsure about vaccination, particularly as the advice has changed since the start of the pandemic.
Expectant mothers were initially advised not have the jab, but the health profession says it's now safe to so do.
"I can fully appreciate why people are nervous. There hasn't been a consistent narrative and when Covid started there was so much uncertainty. Now the message is very clear.
"Pregnancy is such a scary time. You find out and you try to get to 12 weeks and then 20 weeks and you are constantly worried. I think rather than shaming women, encouraging women to talk to their GP or midwife is important.
"When I went for my booking appointment, they asked me if I was vaccinated and I said I was double vaccinated. They were really happy. They almost seemed surprised.
"That's when it struck me, they are dealing with a lot of women who are not.
"I have a responsibility to use my role to promote it. It's so important that anyone with a profile or a platform is making this message clear," she says.
Research from the University of Edinburgh published last week shows that 98 per cent of pregnant women hospitalised with Covid were not vaccinated.
It found that pre-term births, stillbirths and newborn deaths are more common among women who have Covid 28 days or less before their delivery date.
The findings sparked a warning from Northern Ireland's Chief Medical Officer Sir Michael McBride who says vaccination is "crucial in protecting women and babies from the life-threatening complications that can be associated with the virus."
The Royal College of Midwives director Karen Murray says the research shows the vaccine is the best thing women can do to protect themselves and their babies.
"The statistics are stark - the vast majority of pregnant women admitted to hospital with Covid are unvaccinated," she says.
While Lord Mayor Kate's tweet about her vaccination and encouragement of other expectant mothers was largely welcomed, some Twitter users disagreed with her.
One account claimed wrongly that the vaccination is still in its test phase and another posted a link to a government website for vaccine damage payments.
She takes a cautious approach to online comments after receiving sexually explicit and disturbing Facebook messages following her appointment as Lord Mayor last June.
And the politician says she is prepared to engage with anyone but an incident with anti-vaccine protesters last year left her horrified.
"I had a lot of sympathy at the beginning for anti-vaxxers. I have less sympathy now," says Kate.
"During the 9/11 commemoration held with the US Consul General, which was a very moving ceremony, some of the anti-vaxxers who were protesting at the City Hall came over and started screaming abuse. They were attention seekers who saw the cameras.
"There are some people who have genuine concerns and I will always engage with them, and there are some people who are completely irrational.
"We live in unprecedented times and you should always listen and engage with people who have different views.
"I saw at that event there is no way you could engage with people who could shout abuse. They were devoid of humanity and any sense of compassion.
"The things they are saying and doing are so damaging."
She has already made history by becoming the first non-native of Northern Ireland to become Lord Mayor, after moving from Zimbabwe with her parents as a child.
Kate, whose baby is due in May, will make history again as the first person in the role to have a baby in office, but she plans to be on the campaign trail for the elections on May 5.
The politician, mum to two-year-old Cian with husband Fergal, hopes speaking out will reassure other women who are pregnant or planning to have a baby to get vaccinated.
"On social media the evidence can sometimes get drowned out by all the other noise.
"The research from the Edinburgh study has been really clear. Getting a Covid vaccine is one of the most important things a woman can do to stay safe and keep her baby safe," says Kate.