The U.S. government’s plan to vaccinate hundreds of millions of Americans against the coronavirus, potentially save hundreds of thousands of lives and return the country to normalcy may hinge on the effectiveness of an index card.

Two separate vaccines due to be released from pharmaceutical firm Pfizer and biotech Moderna appear to be more than 90% effective in preventing COVID-19. Critically, however, that’s only after people receive two doses taken 21 days apart for Pfizer’s vaccine and 28 days apart for Moderna’s remedy.

The upshot: A major logistical challenge involved in inoculating the bulk of the U.S. population will be to ensure that people return for their second shot. The federal government’s solution: A 4-by-6 index card.

Millions of the cards will be shipped with the vaccine kits sent to hospitals and other distribution centers. Health care providers will fill out the card with “accurate vaccine information,” including a written reminder of the patient’s appointment for a second dose. The Centers for Disease Control and Prevention says providers should encourage patients with smartphones to take a picture of the card in case it gets lost.

But with just a few weeks before the first COVID-19 vaccine becomes available, a number of experts told CBS MoneyWatch they are concerned that the CDC’s measures to ensure people return for a second dose could prove inadequate.

“I hope the CDC has something else planned,” said Mark Fendrick, a medical school professor at the University of Michigan. “But I don’t know of anything other than the index card.”

Soumi Saha, a pharmacist and director of advocacy at Premier, a purchasing agent for hospitals that is also working to develop vaccine distribution plans, told CBS MoneyWatch she is not aware of any national electronic system to track and encourage Americans to return for a second dose. She said hospitals will use their normal appointment scheduling systems, but that will be a hospital-by-hospital effort.

“If you got that dose somewhere else, I am not sure how a hospital would know,” Saha said.

As many as 30% of the patients who get the first dose of a COVID-19 vaccine may not return for a second, estimates Michigan’s Fendrick, an expert in health payment mechanisms and patient incentives. Part of that is human nature. Typically, about a third of patients don’t show up for regular appointments, and as many as half of prescriptions never get filled.

For example, studies show just over 80% of people who get a two-dose vaccine for shingles, a viral infection that can cause a severe rash, typically return for the second treatment. Factor in the side effects associated with the COVID-19 vaccines, which include headaches, fever and muscle pain, as well as misinformation that is likely to circle around the vaccination, and the percentage of people who don’t return for a second dose for the coronavirus shot could be high, Fendrick believes.

“Even without side effects there would be drop-off,” said Fendrick, who has been studying the issue for months. “But one of the anticipated adverse consequences of an effective COVID-19 vaccine is that people — after they take it — are going to feel like crap.”

Nightmare scenario: Mutation

The consequences of people skipping a second vaccine dose could be significant. Although the coronavirus is unlikely to become vaccine-resistant, that could change if millions of individuals only get one dose of a vaccine that requires two treatments, said biologist David Kennedy, who studies viruses at Penn State University and co-authored a recent paper urging drug makers to look for signs of mutation in the coronavirus.

The problem, according to Kennedy: If someone who has had only a single dose is exposed to the virus, their immune system might not be able to kill it off. That could allow the virus to develop a response to the limited immunity provided by that one dose.

“In imperfect vaccines, that’s where we see resistance pop-up,” Kennedy said. “The more individuals who have one dose of these vaccines, the more concerned I would be.”

The CDC’s 75-page playbook for the COVID-19 vaccine program includes only a half-page on its plans to remind people to return for a second dose. Besides the index card, the agency’s blueprint says hospitals and other health care providers also have their own methods for letting patients know about their vaccine appointments. It also suggest robocalls may be the answer.

A CDC spokesperson said the reminder card is just one part of the broader effort to encourage people to come back for a second vaccine dose. The government is also paying providers significantly more — $28 compared with $17 — to administer a second dose than for a first, a financial incentive for providers to successfully convince patients to complete their treatment. That’s on top of the roughly $40 cost per dose of the vaccine, which the government is also covering.

In addition, the CDC says it has a vaccine tracking system that will have the ability to send email and text reminders. That system, though, was built for patients to report side effects, not to remind them about appointments.

A number of experts have suggested that paying individuals to get vaccinated might encourage compliance. In late November, the Centers for Medicare and Medicaid issued a brief note saying it would allow providers to pay out cash rewards or other incentives directly to patients if they get a COVID-19 vaccination. But the government hasn’t provided any additional funding for these incentives.

Where are the funds?

Over the past few years, states have been developing systems to track immunization records, said L.J Tan, the chief strategy officer of the Immunization Action Coalition, a nonprofit that works to increase vaccination rates in the U.S. Many of those systems have the functionality to remind people about future appointments.

But these relatively new systems have only been able to sign up an average of 25% of the nation’s vaccine providers, Tan said. Worse, states lack the funding and staffing to accelerate the process in time for the coronavirus vaccine rollout, he added.

The most recent bipartisan stimulus proposal includes $16 billion in funding for vaccine distribution and contact tracing. But it is unclear how much of that would go to states or whether the funds would arrive in time to be effective.

“We are behind,” Tan said. “The question is do we have enough funding at the state level to get these systems up and running, and I think the answer is no.”