In March 2009, a new strain of pandemic influenza A (H1N1) emerged in Mexico, seriously affecting mainly young children and adults. Although seasonal influenza viruses have been associated with a high rate of hospitalization but low mortality among children, the effect and severity of 2009 H1N1 influenza in children were unknown.
Through a multicenter study in collaboration with public and private institutions in Argentina , INFANT characterized the burden of this new virus in the pediatric population within three months of when the pandemic was announced.
This study, which was published in The New England Journal of Medicine, involved 251 infants and children hospitalized in the network of participating hospitals (coverage area 1.2 million children) with confirmed results of pandemic influenza A (H1N1) by polymerase chain reaction (PCR), revealing the following:
- 75% were <2 years of age.
- The main signs and symptoms present at admission included fever (88%), hypoxemia (82%), cough (70%) and rhinorrhea (32%).
- Blood cultures were positive in only 10 children.
- 32% had one or more pre-existing conditions such as asthma, immunosuppression, chronic lung disease, neurological disease or heart disease.
- 30% lived in overcrowded conditions.
- Ten of 205 children (5%) had received seasonal flu vaccine before the 2009 season.
- 47 (19%) children were admitted to an intensive care unit.
13 (5%) patients died:
- The mortality rate was 1.1 per 100,000 children.
- Death was higher among children under 1 year (7.6 per 100,000).
- 85% were younger than 4 years old.
- 69% had preexisting conditions.
- Most patients died of refractory hypoxemia (62%).
Identifying groups at high risk of mortality
- Children with pre-existing conditions OR = 4.87 [1.3 to 22.2], p = 0.005).
- Children with chronic lung diseases such as asthma (OR = 3.7 [1.0 to 13.6], p = 0.02).
- Children with neurological disorders (OR = 5.6 [1.13 to 22.6], p = 0.003).
Conclusions of the pandemic:
- The 2009 circulation of pandemic influenza virus strain A (H1N1) was atypical in the number of cases and also in the impact and severity of the disease in children.
- Pediatric hospitalization rates were 2 times higher than in previous years.
- Pediatric mortality rates were 10 times higher than in previous years.
- Children under 4 years old and children with asthma or neurological disorders had a marked increased risk of severe disease.
Concrete Impact of these findings on Public Health
- This study provided valuable information regarding the behavior nd impact of this new virus in the pediatric population.
- The findings allowed to anticipate information about the epidemiology and the impact of disease due to influenza in the Northern Hemisphere and also to potentially inform the strains and the effectiveness of prevention measures.
- Identification of groups at high risk for severe disease as priority groups for vaccination and early empiric antiviral treatment.
- This information was essential as a basis for the recommendation of the influenza vaccine for all children under 5 in Argentina and other countries.
After 2009, the year in which they experienced the new pandemic influenza virus, INFANT investigators continued evaluating this disease, and found a significant decrease in the number of cases during 2010 winter season.
It is believed that this phenomenon is due to the high rate of vaccination (93%) in children under 5 years with preexisting diseases, the increased availability of early empiric antiviral therapy and a high rate of infection in 2009 (1 in 3 children acquired the H1N1 influenza virus in that year leaving a large part of the population immunized).
This follow-up study was also published in The New England Journal of Medicine, one of the highest impact scientific journals worldwide.