MILWAUKEE — The Milwaukee Health Department is giving a nudge to those in the city who are still half-vaxxed against COVID-19.

Starting last week, any city of Milwaukee resident who gets their second dose of Pfizer or Moderna at one of the health department’s clinics — the Northwest Health Center, the Southside Health Center, or the recently launched Menomonee Valley COVID-19 site — has been eligible for a $25 U.S. Bank gift card.

“It is crucially important for everyone to complete their COVID vaccination series to be protected from the virus,” Milwaukee Health Commissioner Kirsten Johnson said in a release. “With this incentive program, we hope to encourage those who only got their first shot to prioritize getting their second.”

As of Friday, Milwaukeeans had claimed around 350 of 1,000 available gift cards, health department spokesperson Emily Tau said in an email.

Milwaukee previously offered gift cards as an incentive for people to get their first doses, and gave away all 1,000 rewards within a week. Now, they’re working to fill in some of the gaps for those who are halfway through their vaccine series.

As of last Thursday, 63.3% of adults who live in Milwaukee had gotten at least one dose of a vaccine, while 58% had finished their vaccine series, Tau said. 

Some of those people are still in the waiting period between doses — three weeks for Pfizer and four for Moderna — she said. But that delay doesn’t account for everyone in the city who’s only gotten one shot of a two-dose series.

These second-dose gaps aren’t limited to Milwaukee: Across Wisconsin, more than 3.36 million people have gotten at least one dose of a COVID-19 vaccine, while just around 3.20 million are fully vaccinated, according to DHS data. 

That leaves more than 160,000 residents who are between two doses — and tens of thousands who are past their recommended waiting window.

In fact, across the whole U.S., more than 20 million people got their first dose more than four weeks ago, but haven’t followed up with a second shot, according to CDC data.

There are a few possible reasons why people might be putting off their second doses, said Dr. Mary Beth Graham, medical director of infection prevention and control at Froedtert and the Medical College of Wisconsin.

“I would suspect that they had some type of symptom after the first one and they just don't want to deal with that again,” Graham said. “There also may be an issue of access to care.”

Tau said in an email that Milwaukee’s incentive program is meant to bring in people who “haven’t prioritized getting their second dose” — like those who are concerned about side effects or believe that one dose of mRNA vaccine is enough to protect them.

For most people, though, that second dose serves an important role in helping the body stay ready to fight off the virus, Graham said.

The vaccines work by letting your body get a sneak peek of what the coronavirus looks like. With the mRNA vaccines, Pfizer and Moderna, the shots include the instructions for the virus’s signature spike protein.

Your cells use those instructions to pump out spike proteins — which look like pieces of the virus, but can’t make you sick — so your immune system can recognize them and start building its defenses.

“With the first dose, if you’ve never had COVID-19, your body is learning something brand new,” Graham said. “The immune cells are trying to figure out, ‘Oh, what do I do now?’”

Most people with healthy immune systems produce antibodies to the coronavirus after their first vaccine dose, Graham said. And other parts of the immune system also get a jump start, including the memory cells that help the body remember the virus in the long run.

When that second dose comes around, your body already has an idea of what to expect — and kicks its response into a higher gear.

“Our body will remember having seen something. It gives them an extra boost, and it's like, ‘Oh, make more antibody,’” Graham said. “‘Oh man, we’ve seen this before.’”

If the first vaccine dose helps the body learn the ropes, then the second dose helps it build out a stronger response to fight off infection, she said. 

Clinical trial results from Moderna found that participants had much higher antibody levels after two doses, compared to when they were between their shots. And in Pfizer’s trial data, the efficacy rate against symptomatic COVID-19 sat at around 52% after one dose — but shot up to 95% after two doses.

Of course, some people now have the chance to get a booster dose for extra protection. 

For the mRNA vaccines, the boosters are available for people who are at a higher risk of exposure or severe disease — including older adults and patients with underlying health conditions, who tend to see their antibody levels wane more in the months after their initial two doses. 

Like the second dose, the third booster dose acts as a wake-up call for your body to stay alert against the coronavirus, Graham said. Your immune memory keeps a record of what the virus looks like even after your antibody levels fall, but having more immune fighters at the ready helps the body respond more quickly.

“As the antibodies wane, or you're further and further out from when you first saw it, your body is still going to respond,” she said. “It's just, if you give that boost, then you already have circulating antibody or you already have T cells ready to go — rather than saying, ‘Oh, wait, wait, wait, I remember this, I need to wake up.’”

Even as boosters roll out, health officials have stressed that getting people through their initial vaccine series is still the top priority. 

And for those who missed the recommended three- or four-week window between shots, Graham said the second dose is still effective. In fact, some research out of the U.K. suggested a later second dose could lead to higher antibody levels for some.

“Go ahead and get it anyway,” Graham said. “Because it does not seem to make the antibody response worse. If anything, it’s as good, if not better.”