“My son is sick with the flu at home, and half of his classmates at school are also sick. “In addition, this week one of them was hospitalized in the pediatric ward.” This real-life comment from the mother of a one-year-old child illustrates the concerns many parents have about a disease their children may be protected from. In Spain, in the fall of 2023, free administration of the influenza vaccine to all children aged 6 to 59 months began.
Although vaccination coverage in this population group is not yet available, preliminary information indicates low levels. In fact, they are much lower than desired.
Mutant disease
Influenza is an infectious disease caused by influenza viruses belonging to the family Orthomyxoviridae. Types A and B viruses have surface antigens (hemagglutinin and neuraminidase), which often mutate and give rise to new strains. In temperate countries such as Spain, these variants cause epidemics during the cold season (late autumn and winter).
The mutation phenomenon explains why the disease’s behavior changes each season and why vaccines need to be introduced against the most recently identified viruses.
Almost always soft, but…
In children, influenza usually presents with a sudden rise in temperature, often accompanied by a nonproductive cough, chills, diffuse muscle pain, headache, and general malaise. Respiratory symptoms then appear, including sore throat, nasal congestion, rhinitis and cough. Sometimes the disease also presents with abdominal pain, nausea, vomiting and diarrhea, and infants may suffer from a generalized infection.
Although most children make a full recovery in three to seven days, complications such as pneumonia, ear infections and, less commonly, neurological disorders such as encephalitis, meningitis and seizures may occur. Particularly dangerous is encephalitis, a rare disease with low morbidity but high mortality, especially in children under five years of age.
The proportion of children who become ill each season ranges from 20 to 30%, depending on the circulating pathogens. It is estimated that more than 100 million cases of influenza occur in children under five years of age worldwide each year. Influenza viruses account for 5% of hospitalizations for acute lower respiratory tract infections.
In Spain, surveillance data shows that the hospitalization rate for children under five in the 2019/20 season was 52.4 per 100,000 inhabitants.
Between 2013-14 and 2019-20, 43.8% of patients of this age admitted to intensive care units had one or more risk factors (pulmonary, cardiovascular, renal, hematological diseases, immunosuppression, diabetes or neurological disorders, among others), but a larger percentage (the remaining 56.1%) did not submit them. Of the 16 patient deaths that occurred among children hospitalized during this period, two had no risk factors.
Studies from other countries also highlight that a significant proportion of childhood deaths from influenza occur in previously healthy people.
Vaccines work
Fortunately, we have very valuable tools to address this public health problem: several types of inactivated influenza vaccines (made from inactivated viruses and administered by intramuscular injection) and an attenuated vaccine (made from live pathogens, appropriately attenuated, and administered intranasally).
Results from clinical studies in pediatric populations have shown that the effectiveness of inactivated vaccines ranges from 45% to 91%. These are percentages that depend on the greater or lesser agreement between circulating strains and vaccine strains.
Some authors have suggested that the effectiveness of the attenuated modality is slightly higher than that of the inactivated modality, but a study conducted in Finland on two-year-old children found no difference.
Side effects of inactivated influenza vaccines are transient and mild: 5% to 12% of vaccinated children experience fever, myalgia, fatigue, and pain and swelling at the injection site. In the case of weakened ones, the most common harmful reactions are nasal congestion and fever. However, as with all vaccines, indications must be made by a healthcare professional.
These violations are not contraindications for its use next season. Only children who have had an allergic reaction should be tested to decide whether they should receive the vaccine in other campaigns or not.
Considering the high incidence of influenza among children aged 6 to 59 months, causing hospitalizations, intensive care unit admissions and even death, and the fact that available vaccines have been demonstrated to be effective and safe, it is difficult to understand that vaccination coverage is low.
This situation, similar to that observed in other countries, should force us to rethink strategies to improve the protection of children through vaccination.
The article was written on the recommendation of the Spanish Society of Epidemiology.