Getting the influenza vaccine will be even more important for adults and children this fall because of the Covid-19 pandemic. Flu experts at the Centers for Disease Control and Prevention state that “it’s likely that flu viruses and the virus that causes Covid-19 will both be spreading. In this context, getting a flu vaccine will be more important than ever.” That’s because hospitalizations for influenza can stress hospital capacity even without a pandemic.

But in a disturbing trend, rates of routine childhood vaccination appear to have fallen dramatically in the U.S. during the coronavirus pandemic. That’s understandable: Americans have been asked to stay home and not go to their doctors’ offices unless it was absolutely necessary, though at the same time the CDC, physician groups, and medical practices urged parents to bring in their children to be vaccinated, something that can be done safely during the pandemic.

In a study recently published in the New England Journal of Medicine, we uncovered a new and surprising reason why children may not get the flu vaccinations they need this fall: their birthdays.

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We analyzed more than 1 million children aged 2 to 5 years during the 2015-2016 and 2016-2017 flu seasons in a database of commercial insurance claims, and found that those whose birthdays fell between September and December were significantly more likely to receive the influenza vaccine than those with birthdays in earlier months.

Although that may at first seem odd, it makes sense when viewed through the lens of childhood vaccination schedules.

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Vaccines are typically administered on a schedule that is carefully designed to provide early protection against diseases that can infect and cause serious problems in young children. These vaccines also serve to protect the larger community since young children — who may not cover their coughs or realize that their slobber is infectious — contribute every cold and flu season to the spread of infectious diseases like influenza to other children and to the adults around them.

Fear, mistrust, and misinformation about vaccines have led to children not getting vaccinated, which has triggered subsequent outbreaks of diseases like measles, once considered eradicated in the U.S. But even patients and parents who want recommended vaccines can still face barriers resulting in missed vaccines, such as not being aware of the vaccine schedule, transportation and other factors that make access to health care challenging, work and child care issues, and financial costs, to name a few.

Where do birthdays come in? It’s the month that matters: 53% of kids with September birthdays received the influenza vaccine in the two flu seasons we studied, compared to only 41% of kids with June birthdays. What’s more, those born between September and December were less likely to have been diagnosed with influenza that year, as were their older family members. This means that the additional influenza vaccinations received by children with birthdays between September and December seemed to protect not only themselves but their families.

Of course, there was nothing biologically different about children born between September and December compared to children born in earlier months. But because young children often go for their annual doctor’s visit around their birthdays, kids with birthdays in September to December — when the yearly influenza vaccine becomes available in doctors’ offices — were able to get the vaccine at their annual visit. Kids born in earlier months had to wait until the vaccine was available in doctors’ offices, and then make a special trip to get it.

In the study, the older the children were, the less likely they were to have their annual doctor’s appointment timed near their birthday, to the point that teenagers were no more likely to have their annual visits near their birthday than at any other time. Older kids, over time, have their annual visits drift away from their birthday, coming in at irregular intervals for things like sports physicals. Fittingly, we did not see the birthday-related pattern for influenza vaccination or diagnosis rates for teenagers that we saw in young children.

Our study raises two important questions: Why did we see this pattern of uneven influenza vaccination rates among children, and what should we do to address it?

Unlike older children or adults who may be able to receive their flu shot at a pharmacy or a work/school-sponsored vaccination program, young children in the 2-to-5-year age group generally have to go to their pediatrician’s office to receive their vaccines. This means that kids whose annual visits occur before the year’s vaccine is available must return to the doctor later in the year. As most parents of young children can attest (ourselves included), it would be easy to forget to make that extra appointment and potentially difficult or costly to keep it — even if reminders were sent.

Because receiving the annual influenza vaccine is important for both young children and the public health of their larger communities (by reducing spread of the virus to the vulnerable and elderly), we need creative solutions to make sure that kids with January to August birthdays do not miss getting the influenza vaccine.

Day-care-based immunization programs appear to be safe and effective, yet they have not been widely adopted. Although many community pharmacies now administer vaccines in convenient and lower-cost settings, most states do not allow children under 5 to be vaccinated there, even though this would be a safe option for most children.

All states require proof of basic childhood immunizations before a child enters school, but they have historically not required the flu vaccine.

In a typical year, millions of children in the United States are infected with influenza. In the past decade, the virus has killed several hundred children and caused thousands of hospitalizations. But children are also responsible for spreading the flu and other infectious diseases to older, more vulnerable members of the community who are more likely to need hospitalization or die from the infection. Given the coronavirus pandemic, influenza vaccination should be required for anyone over 6 months of age who will be in close contact with others at day care, school, or college campuses this fall.

Many of the creative solutions that will be needed to widely administer a coronavirus vaccine when it becomes available could ultimately prove helpful more immediately with the flu vaccine.

Taking steps to maximally vaccinate children against influenza is always smart policy, but this fall it will be more critical than ever as the coronavirus continues to circulate. We may not have a vaccine against coronavirus, but we will have one against influenza. Something as simple as a child’s birthday shouldn’t affect the likelihood that he or she or a family member gets the flu. The toll the Covid-19 pandemic will take this winter is hard to predict. Let’s not give ourselves more problems than we already have.

Christopher M. Worsham is a pulmonologist and critical care physician at Harvard Medical School. Anupam B. Jena is a physician, economist, and associate professor of health care policy at Harvard Medical School.

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