The vaccine’s effectiveness against infection in the short term has been established, as has the waning effectiveness of the vaccine over time, wrote Juan Manuel Castelli, MD, of the Ministry of Health of Argentina, Buenos Aires, and colleagues, in the British Medical Journal.
However, data on the impact of vaccine effectiveness on mortality in children and adolescents are limited, especially during periods of omicron variant dominance, the researchers said.
In their new study, the researchers reviewed data from 844,460 children and adolescents aged 3-17 years from the National Surveillance System and the Nominalized Federal Vaccination Registry of Argentina, during a time that included a period of omicron dominance.
Argentina began vaccinating adolescents aged 12-17 years against COVID-19 in August 2021 and added children aged 3-11 years in October 2021. Those aged 12-17 years who were considered fully vaccinated received two doses of either Pfizer-BioNTech and/or Moderna vaccines, and fully-vaccinated 3- to 11-year-olds received two doses of Sinopharm vaccine.
The average time from the second vaccine dose to a COVID-19 test was 66 days for those aged 12-17 years and 54 days for 3- to 11-year-olds. The researchers matched COVID-19 cases with uninfected controls, and a total of 139,321 cases were included in the analysis.
Overall, the estimated vaccine effectiveness against COVID-19 was 64.2% during a period of delta dominance (61.2% in children aged 3-11 years and 66.8% in adolescents aged 12-17 years).
During a period of omicron dominance, estimated vaccine effectiveness was 19.9% across all ages (15.9% and 26.0% for younger and older age groups, respectively).
Effectiveness of the vaccine decreased over time, regardless of the dominant variant, but the decline was greater during the omicron dominant period, the researchers noted. During the omicron period, effectiveness in children aged 3-11 years decreased from 37.6% at 15-30 days postvaccination to 2.0% at 60 days or longer after vaccination. In adolescents aged 12-17 years, vaccine effectiveness during the omicron period decreased from 55.8% at 15-30 days postvaccination to 12.4% at 60 days or longer after vaccination.
Despite the waning protection against infection, the vaccine’s effectiveness against death from COVID-19 was 66.9% in children aged 3-11 years and 97.6% in adolescents aged 12-17 during the period of omicron dominance, the researchers noted.
The results are consistent with similar studies showing a decreased vaccine effectiveness against infection but a persistent effectiveness against deaths over time, the researchers wrote in the discussion section of their paper.
“Our results suggest that the primary vaccination schedule is effective in preventing mortality in children and adolescents with COVID-19 regardless of the circulating SARS-CoV-2 variant,” the researchers said.
Study limitations and strengths
The study was limited by several factors including the incomplete data on symptoms and hospital admissions, the possible impact of unmeasured confounding variables, and the observational design that prevents conclusions of causality, the researchers noted. However, the results were strengthened by the large sample size and access to detailed vaccination records, they said.
Both heterologous and homologous mRNA vaccine schedules showed similar effectiveness in preventing short-term infection and mortality from COVID-19 during periods of differing dominant variants, they noted.
The study findings support the vaccination of children against COVID-19 as an important public health measure to prevent mortality in children and adolescents, they concluded.
Data support value of vaccination, outside experts say
“COVID vaccines may not be as effective over time as the gene variants in the SARS-CoV-2 virus change,” Adrienne G. Randolph, MD, a pediatrician at Harvard Medical School and Boston Children’s Hospital, said in an interview. “Therefore, it is essential to assess vaccine effectiveness over time to look at effectiveness against variants and duration of effectiveness.” Dr. Randolph, who was not involved in the study, said she was not surprised by the findings, which she described as consistent with data from the United States. “COVID vaccines are very effective against preventing life-threatening disease, but the effectiveness against less severe illness for COVID vaccines is not as effective against Omicron,” she noted.
The take-home message for clinicians is that it’s important to get children vaccinated against COVID to prevent severe and life-threatening illness, said Dr. Randolph. “Although these cases are uncommon in children, it is not possible to predict which children will be the most severely affected by COVID,” she emphasized.
However, “we need more data on the new COVID booster vaccines in children that are designed to be more effective against Omicron’s newer variants,” Dr. Randolph said in an interview. “We also need more data on COVID vaccine effectiveness in the youngest children, under 5 years of age, and data on vaccinating mothers to prevent COVID in infants,” she said.
Tim Joos, MD, a Seattle-based clinician who practices a combination of internal medicine and pediatrics, agreed that future research should continue to assess how the new COVID boosters are faring against new variants, noting that the current study did not include data from children who received the new bivalent vaccine.
“The methodology of this study uses a test negative case control design which is common for estimating vaccine effectiveness post-release of a vaccine, but is subject to biases,” Dr. Joos explained. “These are not the clean effectiveness numbers of the prospective randomized control trials that we are used to hearing about when a vaccine is first being approved.”
“Nevertheless, the study reinforces the initial manufacturers’ studies that the vaccines are effective at preventing infection in the pediatric population,” Dr. Joos said in an interview. The current study also reinforces the effectiveness of vaccines in preventing “the rare but devastating mortality from COVID-19 in the pediatric population.”
Commenting on other research showing an increasing ratio of COVID deaths among vaccinated individuals compared to total COVID deaths, he noted that this finding is “likely reflecting a denominator effect of rapidly declining COVID deaths overall,” partly from the vaccines and partly from immunity after previous natural infection.
The study received no outside funding. The researchers, Dr. Randolph, and Dr. Joos had no financial conflicts to disclose. Dr. Joos serves on the Editorial Advisory Board of Pediatric News.