New York State health data collected during the Omicron wave showed while the paediatric Pfizer vaccine prevented severe illness and hospitalisation in children, it offered virtually no protection against infection after around a month, falling as low as 12 per cent.

“In the Omicron era, the effectiveness against cases of BNT162b2 (Pfizer-BioNTech) declined rapidly for children, particularly those five to 11 years,” the researchers from New York State Department of Health and Albany University concluded in a preprint paper uploaded to medRxiv on Monday.

“However, vaccination of children five to 11 years was protective against severe disease and is recommended. These results highlight the potential need to study alternative vaccine dosing for children and the continued importance layered protections, including mask wearing, to prevent infection and transmission.”

The researchers analysed statewide immunisation, laboratory testing and hospitalisation databases covering 852,384 fully vaccinated children aged 12 to 17 and 365,502 children aged five to 11.

The analysis compared outcomes among fully vaccinated children – defined as two weeks after their second dose – versus unvaccinated children in the two age groups.

They found that from December 13, 2021 to January 20, 2022, the vaccine’s effectiveness against infection declined from 66 per cent to 51 per cent for those aged 12 to 17, and from 68 per cent to 12 per cent for those aged five to 11.

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Over the same period, protection against hospitalisation fell from 85 per cent to 73 per cent for 12 to 17s, and from 100 per cent to 48 per cent for five to 11s.

“The finding of markedly lower VE (effectiveness) against infection for children 11 years compared to those 12 and 13 years, despite overlapping physiology, suggests lower vaccine dose may explain lower five to 11 years VE,” the authors wrote.

Pfizer’s vaccine for children five to 11 is 10 micrograms, compared with 30 micrograms for those aged 12 and up.

“Children 12 years had the highest VE of all ages, potentially due to being small size relative to dose and more recent vaccination (by six weeks on average) than those 13-17 years. This gap suggests a threshold effect between the two BNT162b2 vaccine doses and need for study of numbers of doses, amount per dose, dose timing, and/or antigens targeted for children five to 11 years.”

The findings have not yet been peer reviewed.

Last month, Pfizer halted its application to the US Food and Drug Administration to expand its two-dose Covid-19 vaccine to children aged six months to four years.

Pfizer said it was it was awaiting data on a three-dose series instead, saying three doses “may provide a higher level of protection in this age group” and that two doses did not generate a strong enough immune response.

The company said the new data was expected in early April.

According to Health Department figures as of Tuesday, nearly 2.28 million, or just over half of Australian children aged five to 11 have had one dose of a vaccine, while just under 79 per cent, or more than 1.24 million children aged 12 to 15 have had two doses.

The federal government’s vaccine advisory group, the Australian Technical Advisory Group on Immunisation (ATAGI), first recommended the Pfizer vaccine for children aged five to 11 in December.

At the time, ATAGI said it was basing its recommendation the “direct benefits for the child in preventing illness” and the “indirect benefits” such as preventing school closures.

Noting “vaccine effectiveness data from real-world experience are not yet available for children aged five to 11”, ATAGI cited one small clinical trial conducted by Pfizer which found three cases of Covid-19 in the vaccine recipient group compared with 16 cases in the placebo group.

“The three cases in the paediatric Pfizer Covid-19 vaccine group were mild and without fever, whereas most cases in the placebo group had documented fever,” ATAGI said. “There were no cases of severe Covid-19 in either group.”

ATAGI acknowledged that the clinical trials were “conducted prior to the emergence of the Omicron variant, and the results reflect vaccine efficacy against older strains of SARS-CoV-2”.

“Vaccine efficacy or effectiveness against the Omicron strain in children aged 5-11 years is not yet known,” it said.

News.com.au requested comment from ATAGI on whether it would reassess its recommendation in light of the new data, and whether federal health officials have conducted any comparable analysis in Australia.

“ATAGI continues to monitor evidence emerging from medical research on the effectiveness of the Covid-19 vaccines both locally and internationally,” a Health Department spokesman said in a statement on Thursday.

“To date, more than 54.2 million Covid-19 vaccinations have been administered across Australia and over 11.6 million Australians have received a Covid-19 booster. Vaccination assists in reducing the likelihood of severe Covid-19, hospitalisation and death. We continue to encourage all eligible Australians to be vaccinated.”

As of February 20, the Therapeutic Goods Administration has received 3600 reports of adverse events from about 3.4 million doses of Pfizer and Moderna in children aged 12 to 17.

“The most commonly reported reactions are chest pain, headache, dizziness, nausea and fever,” the TGA said in most recent safety report.

The medicines regulator has also received 715 reports from about 1.1 million doses of Pfizer in children aged five to 11 – including a possible case of “mild pericarditis” in a 10-year-old boy.

“The most common reactions reported included chest pain, vomiting, fever, fainting and headache,” the TGA said.

“We have received 10 reports of suspected myocarditis and/or pericarditis in this age group. Following review of information in the reports, none were likely to represent myocarditis. One report in a 10-year-old boy possibly represented mild pericarditis when assessed against internationally accepted criteria for this condition.”

frank.chung@news.com.au