Pfizer COVID Vaccine Less Effective in 5- to 11-Year-Olds, Study Shows

Damian McNamara

March 01, 2022

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

You may have already seen the headline — Pfizer's vaccine effectiveness against COVID-19 infection drops to 12% in 5- to 11-year-olds — but how do experts interpret this and other results from this pre-print study?  

Despite the findings, which have not been peer reviewed, researchers at the New York State Department of Health, who conducted the study, say vaccination of children 5 years and older remains essential to prevent severe COVID-19 disease.

The study data shows that "COVID-19 vaccines reduce the risk of more severe illness and hospitalization for children 5-11," New York State Department of Health Commissioner Mary Bassett, MD, MPH, said in a statement.

"I encourage parents and guardians to consult their pediatrician about getting their children vaccinated, and boosted if eligible, as soon as they can," added Bassett, a study co-author.

Adam Ratner, MD, MPH, a member of the American Academy of Pediatrics Committee on Infectious Diseases, says doctors need to talk with patients and parents to make sure this study doesn't discourage them from getting vaccinated.

Part of that talk would be telling them vaccination "is still the best option that we have right now for protection of kids in that age range,” Ratner says.

Although hospitalizations of children during the Omicron surge was rare compared to the rate in adults, he says, the majority of admissions were among unvaccinated children.

One Third the Dose

It could be the dose, not the vaccine, per se, behind the drop from 65% to 12% effectiveness of the vaccine during the Omicron surge. Children in this age group typically receive one third the dose, or 10 micrograms, per injection, compared to a 30 mg dose for adults and children 12 years and older.

"The data are not surprising as the vaccine was developed in response to an earlier COVID-19 variant, and reduced effectiveness of two doses against the Omicron variant has been seen to some degree with all vaccines and ages," Bassett said.

The results were published Monday as a pre-print study on MedRxiv. Pre-prints carry a caveat that the information "has not been certified by peer review. It reports new medical research that has yet to be evaluated and so should not be used to guide clinical practice."

Putting the Onus on Omicron

Previous studies of children 5-to-17 support the safety and effectiveness of the Pfizer COVID-19 vaccine, the researchers point out. The research, however, was conducted before predominance of the Omicron variant.

One study from Pfizer during the Delta surge, for example, reported 90% vaccine efficacy among 5- to 11-year-olds.

Bassett and colleagues looked at infection and hospitalization numbers in New York state starting in late November. They compared these COVID-19 cases between both fully vaccinated and unvaccinated children ages 5 to 11 and 12 to 17 using a state surveillance database.

In the 12- to 17-year-old group, for example, vaccine efficacy dropped from 85% on Nov. 29 to 51% on Jan. 24, 2022, when Omicron accounted for 99% of circulating virus strains.

In the 5- to 11-year-old group, vaccine efficacy decreased from 68% the week of Dec. 13, to 12% by Jan. 24.

Vaccine efficacy was stronger against hospitalization than for preventing infection. The rate of hospital admission was 73% in the older age group and 48% in the younger children.

The proportion of fully vaccinated children in New York was higher in the older age group, 62%, by the week of Jan. 24-30, 2022, compared to 23% of the 5- to 11-year-old children.

Although 11- and 12-year-olds are only one year apart, their recommended vaccine dosages are very different. A "threshold effect" between these groups points to a role of the lower dose in the lower effectiveness, the researchers note. The week of January 24, 2022, for example, vaccine efficacy was 67% for 12-year-olds versus 11% for 11-year-olds.

Given there is no large biological difference between an 11- and a 12-year-old, "you would expect those numbers to be very, very close," said Ratner, who is also Chief of the Division of Pediatric Infectious Diseases at Hassenfeld Children's Hospital at NYU Langone in New York City.

"The only difference that you can really point to there is that kids up to the age 11 got a lower dosing."

'Best Tool We Have'

The researchers note that their data "support vaccine protection against severe disease among children 5-11 years, but suggest rapid loss of protection against infection, in the Omicron variant era. Should such findings be replicated in other settings, review of the dosing schedule for children 5-11 years appears prudent."

"The vaccine is the best tool we have and it's way, way better than not vaccinating," Ratner said. "And it's certainly safe. There is nothing that would indicate any kind of safety issue for 5-to-11 year olds."

In addition, vaccinating children 5 years and older makes even more sense now, he added, that mask mandates and other protections are easing.

The study was funded by the New York State Department of Health and the authors had no relevant financial relationships to disclose. Dr Ratner had no relevant disclosures.

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology, and critical care. Follow Damian on Twitter: @MedReporter.

Sources

New York Department of Health: "Statement from New York State Department of Health Commissioner Dr Mary Bassett on Latest Data On COVID-19 Vaccine Effectiveness.”

Adam Ratner, MD, MPH, American Academy of Pediatrics, Committee on Infectious Diseases.

medRxiv: "Effectiveness of the BNT162b2 vaccine among children 5-11 and 12-17 years in New York after the Emergence of the Omicron Variant."

MedRxiv. Published online February 28, 2022. Abstract

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