Model Estimates Hepatitis A Transmission in Michigan

Image of a vaccine

New research from Andrew Brouwer

Assistant Research Scientist of Epidemiology

Since the introduction of a vaccine in 1996, incidence of hepatitis A has fallen to historic lows. However, recent outbreaks in Michigan beginning in 2016 have researchers eager to learn more about the spread of the virus and investigate the impact of vaccination clinic campaigns to help vaccinate high-risk populations.

We spoke to Andrew Brouwer, assistant research scientist in the Department of Epidemiology at the University of Michigan School of Public Health, to learn more about his findings in a new paper published in Epidemiology.

We have had a vaccine for hepatitis A for over two decades. Why did these outbreaks happen now?

Since the vaccine was introduced, most outbreaks have been foodborne, often from a single source, with little-to-no secondary transmission. These recent outbreaks in 2016, which have affected more than 30,000 people across the US, have been driven by person-to-person transmission among high-risk groups, such as people experiencing homelessness or abusing substances. For me, these outbreaks are an important reminder that just because a vaccine exists, doesn’t mean everyone has access to it or will choose to get it.

You developed a mathematical model to help determine the estimated rate of transmission. Can you talk a little bit about the data that went into this model and what you found?

Michigan maintains a database called the Michigan Disease Surveillance System, which records incidence of reportable diseases, including hepatitis A. The Michigan Department of Health and Human Services attempts to follow-up with every reported case of hepatitis A to record their risk factors and outcomes. 

A second database, called the Michigan Care Improvement Registry, records information about vaccinations. We combined these disease and vaccination data with a transmission model that turns our assumptions about disease transmission and progression into mathematical equations. These equations allowed us to provide estimates of how the outbreak unfolded—and how it might have unfolded differently without vaccination campaigns coordinated by local and state health departments to educate the public about the vaccine.

What was the impact of the vaccination campaigns that were implemented across the state?

The epicenter of the outbreak in Michigan was in the Detroit area of Southeast Michigan, and from there it jumped to other areas of the state. We found that the vaccination campaign in Southeast Michigan did not substantially impact the course of the outbreak there. We think that this was due to a combination of the speed of the outbreak and the difficulty of targeting vaccines to high-risk populations. However, because the outbreaks in the rest of the state were seeded later and were slower to take off, we estimated that those interventions prevented much larger, prolonged epidemics. 

What is the importance of targeting populations where local transmission of hepatitis A is not yet sustained?

It is much harder to intervene once transmission is sustained in a community, particularly when most transmission occurs before symptoms arise and when the people most at risk are the hardest to reach. If we can intervene early and trace people’s contacts, we have the opportunity to prevent further spread.

Can you explain the benefits of proactive measures to control future outbreaks compared to reactive measures for hepatitis A?

Interventions are hard, costly, and take time to implement. Proactive measures—such as routine vaccination—work well and are cost-effective. Although Benjamin Franklin was referring to fire safety when he said, “an ounce of prevention is worth a pound of cure,” the saying is just as true for public health. 

Earlier, I said that the vaccination efforts in Southeast Michigan did not have much of an impact on this outbreak, and that’s likely right. However, those efforts have likely prevented the next potential outbreak.

What lessons should we be learning from outbreaks like hepatitis A and COVID-19?

Public health is often overlooked precisely because it is so effective. Vaccine hesitancy is on the rise in part because we can look around and not see the devastating effects of childhood diseases. When new diseases arise, we assure ourselves that modern medicine can quickly develop a cure for anything. But, as these recent outbreaks demonstrate, we are in danger of losing hard-won gains if we don’t reinvest in public health infrastructure and preparedness.