A new study published in JAMA Pediatrics shows that children who are vaccinated against measles between 12 and 15 months are less likely to suffer fever or seizures than those vaccinated between 16 and 23 months.
The Centers for Disease Control and Prevention (CDC) recommends a first dose injection with a measles-containing vaccine at 12-15 months, with a follow-up "booster" between the ages of 4 and 6.
Approximately 85% of children will have received their first dose by the age of 19 months, but the study found that receiving the first dose by 15 months provides a benefit to children.
Researchers from the University of Washington carried out a retrospective cohort study at eight Vaccine Safety Datalink sites on a total of 840,348 children aged 12 to 23 months of age who had been vaccinated against measles from 2001 through 2011.
Dr. Ali Rowhani-Rahbar, from the University of Washington's Department of Epidemiology and lead author of the study, says:
"We found that the magnitude of increased risk of fever and seizures following immunization with measles-containing vaccines during the second year of life depends on age.
While measles-containing vaccines administered at 12-15 months of age are associated with a small risk of fever and seizures following immunization, delayed administration at 16-23 months of age results in a greater risk of those adverse events."
The CDC estimates that on average, 60 people in the US are reported to have measles each year. Numbers recorded for this year, from January 1st to August 24th were higher - with 159 reported cases.
Children receiving the first dose of the measles vaccine by 15 months reduced the risks of fever and febrile seizures.
Improved vaccination programs have dramatically reduced the number of people suffering from measles - according to the CDC, by more than 99%.
Measles is still common in other countries. The virus is highly contagious and can spread rapidly in areas where vaccination is not widespread. The CDC estimates that in 2008 there were 164,000 measles deaths worldwide - that equals about 450 deaths every day or about 18 deaths every hour.
And in April 2013, Medical News Today reported up to 2 million kids in the UK are at risk from the disease because of low uptake of the vaccine 10 years ago.
Seizure risk still low
Previous studies have shown that measles vaccines given to children 12-23 months of age are associated with an increased risk of febrile seizures 1 to 2 weeks after immunization. This is the period of time during which the vaccine virus replication is at its peak, potentially causing fever. The resulting fever may cause some children to have a seizure - a potentially traumatic experience for parents or caregivers.
The CDC states that during the 7-10 days after vaccination, about one additional febrile seizure would be expected to occur among every 2,000 children vaccinated with the measles, mumps, rubella and varicella (MMRV) vaccine, compared with children vaccinated with MMR and varicella separately at the same visit.
While febrile seizures are the most common adverse effect following immunization with measles-containing vaccines, senior author and co-director of the Vaccine Study Center Dr. Nicola Klein notes that the risk is small regardless of age:
"Medically attended febrile seizures following immunization with measles-containing vaccines are not common events. Concerned parents should understand that the risk for febrile seizures after any measles-containing vaccine is low - less than one febrile seizure per 1,000 injections."
Modifying effect of age on reaction to vaccine
Using data from the Vaccine Safety Datalink, researchers from the CDC and nine managed care organizations evaluated the potential modifying effect of age on the risk of fever and seizures following immunization using different combinations of vaccines:
- Any measles-containing vaccines
- The MMRV vaccine compared with the measles, mumps and rubella vaccine (MMR) administered with or without a separate varicella vaccine (MMR+V).
Researchers found that while the incidence of fever and seizures during days 7-10 was significantly greater than any other time in all age groups, the patterns for fever and seizures were different.
- Seizures were most common among children aged 16-18 months. The incidence of fever, however, steadily declined from 12-13 to 19-23 months.
- Although there was only a slight risk of fever or seizure during the 7-10 day interval, this risk was significantly greater among the children aged 16-23 months than those aged 12-15 months.
- The risk of seizures was also significantly greater among children aged 16-23 months than among children aged 12-15 months during the 7-10 day window.
The researchers also note that the incidence of fever and seizures during the 7-10 days following immunization with MMRV was significantly greater than that following immunization with MMR+V. This finding is consistent with the findings from previous studies.