MH: Vaccines are tiny public health heroes. They work by activating the body’s immune system to help build immunity. After vaccination, the immune system produces antibodies that remain in the body to fight against exposure to the virus in the future. Instead of getting the illness itself to develop immunity, the vaccine triggers the same immune response process. The vaccine doesn’t cause disease; it simply teaches the immune system to recognize and quickly react to the germs (virus or bacteria) to prevent serious illness. In other words, vaccines train the body to protect itself against specific germs without the risk of an actual infection. 

Vaccines also work to protect those who cannot be vaccinated. When enough individuals are vaccinated, the ability of disease to spread from person to person is decreased, also known as herd immunity in epidemiology. This herd immunity protects individuals who are too young or immunocompromised, or otherwise unable to receive a vaccine. 

Vaccines undergo rigorous testing to make sure they are safe and effective and are monitored before being authorized for use. Over just the last 50 years, vaccines have saved an estimated 155 million lives, most of whom were infants.  

PH: Why do some vaccines last a lifetime while others, like flu shots, need to be updated or repeated regularly?  

MH: All vaccines trigger immunity, but the length of that protection depends on three factors: how rapidly the virus mutates, the technology used to make the vaccines, and how the immune system remembers the pathogen (how many antibodies are produced, in general, with each dose). 

We only need the polio vaccines once, even though it is highly contagious, because it is a stable virus that does not change over time. The flu virus, however, mutates rapidly, creating new variants that can evade the immune system, known as antigenic drift, which is why this is a seasonal vaccine. We don’t even need to be vaccinated against smallpox anymore because it was eradicated through successful vaccine efforts.  

PH: This flu season, it felt like almost everyone got sick. Is one reason that the flu vaccine didn’t match one of the strains going around (the K strain)? But what does it mean when a vaccine “mismatches” a virus—and does it still help protect you? 

MH: We are in the midst of a brutal flu season. The official flu season lasts until May in the United States. One strain of flu, Influenza A H3N2, mutated over the summer as it spread in the southern hemisphere. This subclade K, the mutated H3N2 has been responsible for approximately 80 percent of the flu cases tested so far.  

The flu virus is well known for quick mutations. The current vaccine covers three strains of the flu. Selecting vaccine strains for rapidly changing viruses, like flu or Covid-19, is both an art and a science. What we are seeing this year is a drift, that is incremental changes that happen as the virus spreads. Unfortunately, the timing of this mutation occurred just before the US flu season, which means the current vaccines, which formulas were finalized in February of 2025, recognizes some but not all of this mutated K strain flu virus. So, this flu virus is better at evading the vaccine and prior immunity from flu infection. And that results in more flu cases, and more severe cases among those at highest risk.  

However, vaccination still matters, and it’s not too late to get your flu shot. The final vaccine formula may not always align with the circulating virus, but it’s still considered an effective vaccine. The current flu vaccine does reduce hospitalization due to severe disease and offers protection again the other still circulating flu strains. On average, flu vaccines reduce the risk of needing to go to the doctor by up to 60 percent. Despite recent changes to the Health and Human Services (HHS) childhood vaccination schedule, the American Academy of Pediatrics and many states, including Maryland, still recommend the flu shot for everyone six months and older.  

PH: If vaccines are one of the most studied medical tools we have, why do misconceptions about them spread so easily?  

MH: Getting vaccinated requires trust in the institution recommending, or in some case requiring, the vaccine to be safe, effective and without significant risks. When that trust is eroded, skepticism towards government, pharmaceutical companies, or mainstream medicine, there is an opportunity for misconceptions to be spread. Social media facilitates easy spread of all information. Social media also makes it very easy to continue to seek, and to receive, information that confirms existing beliefs and may amplify misconceptions.  

I think there’s also a perception problem about some vaccines that is an unintended consequence of their success. When diseases are rare (due to vaccine protection), the perception is that the risk of disease is low and perceptions that vaccine may not be necessary. This is also an ongoing tension and cultural shift in this country on the role of individualism vs collective good that is at odds with the overwhelming scientific evidence on the safety and effectiveness of vaccines. Many misconceptions about vaccines, those that are new since the pandemic and those that have persisted for decades, are due to not understanding how beautifully vaccination works to prevent disease and suffering, and to save lives. 

PH: I keep reading headlines about measles coming back because people aren’t vaccinated. What can happen when people stop getting vaccinated for diseases like measles, mumps, and other diseases that we haven’t seen for a long time in the United States?  

MH: First, you can expect to see outbreaks in communities as herd immunity is lost. Measles and mumps, for example, are highly contagious and if people stop getting vaccinated then the most vulnerable groups (infants, elderly, immunocomprised) are at severe risk of illness. Then we would see measles epidemics of millions of cases and hundreds of completely preventable deaths in this country. And then the numbers of cases and deaths would continue to rise as susceptibility increased and transmission spread.  

We can look to history of pre-vaccine eras when large numbers of people contracted diseases, some with serious complications and deadly outcomes, that can now be prevented. A core principle of immunization is that it’s better to prevent illness than to try to treat or cure it once someone is sick.  

About Melissa Hawkins 

Melissa Hawkins, PhD, MHS, is an associate professor and associate chair in American University’s Department of Health Studies. An epidemiologist, she researches maternal and child health and the integration of Community Health Workers into U.S. health systems. She has led epidemiological studies on women’s and children’s health and earned her PhD and MHS in Public Health from Johns Hopkins University.