Vaccinated parents may shield their kids from COVID-19. Protection is higher for 2 vaccinated parents than one.

Nurse Alyssa Barron, left, administers a test swab to Atly Acevedo, 2, held by mother Jenae Acevedo, 32, at a COVID-19 testing site in Rancho Cucamonga, California, on December 30, 2021.

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• Vaccinated parents may indirectly protect kids from a coronavirus infection, new research suggests.
• An Israeli study found that vaccines and boosters lowered a parent's infectiousness within the home.
• The data was collected during the Alpha and Delta waves, but results may also hold true for Omicron.

Parents who've had a COVID-19 vaccine may indirectly protect their kids from a coronavirus infection, according to a study published Thursday in the journal Science.

The research, part of a collaboration between Harvard Medical School, Ben Gurion University, and Israel's largest healthcare organization, Clalit Research Institute, looked at households with unvaccinated children from January through September 2021 in Israel — before the Omicron variant was detected.

Parents who'd received either two or three doses of Pfizer's vaccine conferred substantial protection to kids living in the same household during that time.

"Parents seem to be maybe the most important vector of infections into the house," Noam Barda, a lecturer at Ben Gurion University and one of the study's authors, told Insider.

The researchers studied two time periods: First, they looked at around 155,000 households with unvaccinated children under age 16, between January and March 2021, when the Alpha variant was dominant. During that time, having one vaccinated parent was associated with a 26% lower risk of children getting infected, relative to an unvaccinated parent. Two vaccinated parents were associated with a 72% lower risk.

Next, the researchers looked at nearly 77,000 households with unvaccinated children under age 11, between July and September 2021, when the Delta variant was dominant. Having a boosted parent was associated with a 21% lower risk of children getting infected, relative to a parent who received two vaccines doses. Having two boosted parents, by contrast, was associated with a 58% lower risk for kids.

The data "makes for a very strong public-health statement," Barda said, adding, "If you want to protect your kids, you've both got to be vaccinated."

But parental vaccination isn't a replacement for kids getting shots themselves, he added. 

"I have two kids and I very much do not want them to be infected unvaccinated," Barda said. "If we are all doomed to be infected, we should do the best we can to [avoid infection until] after we're all vaccinated, because there is good evidence that that makes for milder disease, but probably also lowers the probability of long-term complications."

Vaccines made parents less infectious, but researchers don't yet understand why

Students return to school in New York City.

Jennifer Peltz/AP

Two doses of Pfizer lowered the risk of a parent getting infected by 94% during the Alpha period and 86% during the Delta period, according to the study. The shots also seemed to lower a parent's infectiousness once they got sick, meaning parents were less likely spread the disease to others. 

During the Alpha period, two vaccine doses lowered a parent's infectiousness in their household by 72% compared to an unvaccinated parent. And during the Delta period, a boosted parent's infectiousness went down by 80% compared to a parent who'd had two vaccine doses.

Other research has similarly shown that vaccines lower in-household transmission. A separate study published Thursday in the journal Science found that two doses of Pfizer's vaccine were 7-23% effective at preventing an infected person from spreading the virus to their household contacts, depending on when the second dose was administered. The data was collected before the Delta variant emerged.

Researchers are still trying to understand why vaccinated people tend to be less infectious, on average. Though some studies have found no significant difference in peak viral loads between vaccinated and unvaccinated people, those who are vaccinated seem to clear the virus faster, and therefore might be infectious for shorter periods of time.

"We see that the infected parent is less infectious to the children if he's vaccinated. Does it happen thanks to lower viral load? Maybe," Barda said, adding, "It's perfectly possible."

Omicron may lower indirect protection from parent to child

A father and daughter hold hands.

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Since the Israeli study didn't look at vaccine protection during the Omicron period, Barda said, it's hard to know how well parental vaccination protects kids right now.

"The most reasonable guess — and I'm emphasizing it's a guess — is that we'll see something similar. There's going to be indirect protection," Barda said. But that protection is probably lower, he added, since Omicron renders our current vaccines less effective compared with Delta or Alpha.

The second Science study published Thursday found that waning vaccine immunity has made it harder to prevent coronavirus transmission over time. Transmission also depends on how often people gather together — particularly in crowded, indoor environments.

Barda said it's possible that parental vaccinations wouldn't protect kids as much during times with fewer lockdown restrictions or more exposure outside the home. During the first period of the Israeli study, when Alpha was spreading, schools across the country were shut down. Many students were also on summer vacation during the Delta period, so they likely spent more time at home.

Still, Barda said, his study underscores the importance of COVID-19 vaccines for parents. 

"We still to this day cannot vaccinate under the age of 5 because it's not authorized," he said. "Both parents should, if they can, be vaccinated just to offer that indirect protection."

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