The Centers for Disease Control and Prevention (CDC) has a data problem and it can risk upsetting the fight against COVID-19 if unaddressed.

On Wednesday, experts expressed concern about the accuracy of the vaccination data collected by the CDC, particularly for seniors aged 65 or older. According to the CDC’s COVID Data Tracker, an impressive 99.9% of American seniors have received at least one dose of a COVID-19 vaccine.

Seniors were the first group to receive access to the vaccines when they first became available and providing vaccinations to this high-risk group was an early priority. The same has also been true for booster shots once they became available in September. With this in mind, it was inevitable that this number would be higher than for younger age groups, there is reason to believe that the 99.9% figure is inaccurate.

For one, it is inconsistent with data collected elsewhere, including in the individual states and in separate surveys. For example,the CDC’s data shows that 21 states have vaccinated 99.9% of their senior population, but this figure is not reflected in the health data collected by several of the states in question. California, for example, reported that 86% of its 65 years and older population is vaccinated, yet the CDC lists them in the 99% category. Adding to the discrepancies, surveys from non-governmental organizations have similarly found lower figures for vaccinated seniors.

Experts who spoke to NBC News warn that these inaccuracies can hobble policy responses and planning because it obscures the actual picture for vaccinations among U.S seniors or future efforts to vaccinate a greater share of the U.S population.

“You want to know the best data to plan and prepare and know where to put resources in place — particularly in places that are grossly undervaccinated,” Howard Forman, a professor of public health at Yale University School of Medicine told NBC.

The CDC itself acknowledges that it encounters difficulties when compiling its vaccination data and that it has limitations.

“There are challenges in linking doses when someone is vaccinated in different jurisdictions or at different providers because of the need to remove personally identifiable information (de-identify) data to protect people’s privacy. This means that, even with the high-quality data CDC receives from jurisdictions and federal entities, there are limits to how CDC can analyze those data,” said a footnote on the agency’s data tracker webpage.

There are some efforts underway to remedy this situation. For instance, health officials in Pennsylvania have announced that they have been working with the CDC to correct its vaccination rate figures and are working to avoid duplication between the two datasets.