Inside Baseball: The Developmental Window

Should you vaccinate your infant against measles early?

North Carolina’s Department of Health and Human Services recently updated their recommendations for Measles, Mumps, and Rubella (MMR) vaccination in infants.

In the US, the MMR vaccine is routinely given in two doses at 12 months (MMR-1) and between 4 to 6 years of age (MMR-2). The state’s guidance now recommends that physicians consider giving an additional early dose (MMR-0) between 6-11 months to infants who live in—or are traveling to—counties with community transmission.

My county made the list.

I routinely care for infants in my family medicine practice. I also have a 6-month old at home, so understanding the data became a priority to me as both a physician and a parent.

And as a parent who planned to vaccinate anyway, following the guidance seems reasonable enough. If there have been cases in the area, why not go ahead and get the vaccine a few months early?

But like most decisions in medicine, it’s not as straightforward as it seems.

This post is a snapshot of how I am actively reasoning through this decision for both my son and my patients—an “inside baseball” look, if you will.

Here’s the decision in a nutshell

Measles cases in North Carolina have been rising—23 confirmed new cases in the last three months, with a larger outbreak in neighboring South Carolina counties. More cases are expected.

It’s a dangerous disease: one in five children are hospitalized, and one to three per thousand cases are fatal. Severe neurologic complications, such as encephalitis or acute disseminating encephalomyelitis, occur in 0.1% of cases. A lesser-known complication is subacute sclerosing panencephalitis—a rare (approximately 4-11 cases per 100,000), but debilitating condition that presents 8 to 11 years after infection and causes progressive neurocognitive deterioration.

Infants younger than 12 months are at higher risk for these complications. Waiting to vaccinate carries real risk.

Simultaneously, there are risks to early vaccination.

First, it may not be highly protective. Circulating maternal antibodies may prevent the vaccine from stimulating an immune response.

Second, there is evidence, albeit limited, that infants who receive early vaccination may demonstrate a precipitous drop in antibodies over time—potentially resulting in inadequate protection.

There are legitimate risks on both sides—risks across different time horizons and involving different types of uncertainty.

It’s actually the kind of decision that baseball organizations make frequently.

A question of timing and development

Deciding when to promote a young prospect to the big leagues is exceedingly difficult.

The crux of the issue is developmental risk. Bring them up too early and you risk them struggling at a formative stage. Some young players handle this just fine, while others never recover.

The problem is you don’t know which type of player you have in advance.

Jackson Holliday, the top overall draft pick in 2022 and the son of a seven-time All-Star Matt Holliday, seemed to be a “can’t miss” prospect.

Having successfully fast-tracked top prospects Adley Rutschman and Gunnar Henderson in recent years, the Baltimore Orioles attempted to replicate that success with Holliday. The 20-year-old was promoted on April 10, 2024 with only 18 Triple-A games under his belt.

In his first ten games, Holliday struggled mightily. He went 2-for-34 and struck out 18 times.

His first Major League stint lasted only sixteen days.

He got a second chance in July, but he ended his rookie season with a dismal slash line of .189/.255/.311. Later, Baltimore general manager Mike Elias expressed regret over his decision to fast-track Holliday:

“I think some of our call-ups that (sic) didn’t go smoothly, Jackson being one of them, we didn’t give them enough time in Triple-A.”

Holliday showed improvement in 2025, but his below-average production suggests the early promotion may have left a mark—whether he reaches his ceiling remains genuinely uncertain.

The decision Elias faced had the same structure as the one facing physicians and parents right now. But as we’ll see, the feedback is harder to come by.

Short-term risks of waiting to promote:

• The Orioles, having won 101 games in 2023, appeared to be in the midst of a competitive window.

• Fast-tracking Rutschman and Henderson worked out well. Repeating that success with Holliday would solidify the Orioles as a World Series contender.

• They could miss this rare opportunity by being too cautious with their top prospect.

Long-term risks of early promotion:

• Exposing an immature player to failure at a formative moment may negatively impact his future trajectory.

• They had little data to go on—just a small sample of success at lower levels of competition.

• A repeat of the team’s success from 2023 was not a foregone conclusion. Injuries, poor performance, and bad luck could keep them out of playoff contention regardless.

Short-term risks of waiting to vaccinate:

• Measles in a 6-month-old is genuinely dangerous, with higher rates of severe complications than in older children.

• Cases are already confirmed in the area—exposure risk is real and growing. However, exposure before 12 months is still unlikely at current rates.

• We could miss the opportunity to prevent severe illness by being too cautious.

Long-term risks of early vaccination:

• Early vaccination may blunt the immune response to later doses, potentially leaving children with diminished protection years down the road.

• The evidence supporting this concern comes from a small cohort on a different vaccination schedule than the one used in the US.

So what does the evidence actually say about these risks?