With the covid-19 pandemic still forcefully hitting the United States, voters have been shifting to voting by mail, hoping to avoid transmission. In presidential primaries held since mid-March, voters have cast ballots by mail at historic rates, at up to 10 times the usual rate.

This shift has risks, although not necessarily the ones most widely discussed. President Trump has asserted by tweet that mail ballots are subject to rampant fraud and that mail ballots favor Democrats, both claims disproved by research. And with a long-term decline in service, the U.S. Postal Service has warned that some ballots may arrive too late to count.

But voting by mail has some other practical risks that few have noted. It’s more complex than voting in person, administratively speaking. As a result, those who cast their ballots by mail might be less likely to have their votes counted than those who vote in person.

What are the risks of voting by mail?

In an article forthcoming in the Harvard Data Science Review, I have worked to quantify how much riskier it is for someone to vote by mail than in person. Depending on the state in which a citizen is voting, the increased risk of having your vote lost — meaning, not counted in the election — ranges from 3.5 percent to 4.9 percent.

“Lost votes” is a term coined by the Caltech/MIT Voting Technology Project (VTP) in its 2001 report “Voting: What Is/What Could Be.” Here’s what it means. Suppose a voter wakes up on Election Day, fully intending to vote, and does everything required to do so. If the intention is thwarted, that is a lost vote. For instance, she might arrive at the polls at 6 p.m. and finds the line is so long that she leaves and can’t return to cast a ballot by 7 p.m. when the polls close.

If she intends to vote by mail, here’s what might go wrong. In states where a voter must apply for a mail ballot, the ballot application could get lost in the mail; the local election office could lose the application or deny it; the ballot might not make it back to the voter, for instance, getting lost in the mail; and the marked ballot might not make it from the voter back to the local election office. Even if the ballot arrives, it could be rejected because it arrived late or lacked a signature — the two most common reasons for rejection. Finally, the ballot could have an error that she could have caught had she voted in person.

Here’s how I came up with my estimates

Estimating these risks is difficult. It’s nearly impossible to find hard data on whether the ballot request makes it to the local election office. I approximated this by relying on USPS performance reports, which indicate that approximately 0.4 percent of first-class mail and 0.7 percent of marketing mail fail to reach its destination within three days of the delivery standards set by the Postal Service. I assumed that 99.9 percent of the time, when local election officials do receive a valid application, they respond by mailing out a ballot. If we apply the 0.4 percent failed delivery figure both to the voter’s application for and return of a mail ballot, and then add 0.1 percent chance of it getting lost in the office and the 0.7 percent failed delivery figure for the election office’s response, this suggests that 1.6 percent of mail-ballot applications fail to deliver a ballot to the voter.

We do know what happens once ballots reach election offices, from U.S. Election Assistance Commission data. In 2016, in states that require an excuse to vote absentee, elections offices rejected 1.8 percent of all mail ballots they received. However, in the three states that voted only by mail, only 0.9 percent of ballots were rejected.

Finally, when the mail ballots are scanned, they may be rejected for containing votes for more or fewer candidates than allowed, which could have been detected if the ballot had been scanned in person. In prior research I have conducted with colleagues into absentee voting in California and nationwide, I’ve found that mail ballots are rejected for this reason at rates 1.5 points higher than ballots cast in person.

How this risk varies by state

In states that automatically mail ballots to all voters, the first risk — that a voter’s application won’t be received, or if received, that a ballot won’t arrive — essentially goes to zero. Further, states that require voters to apply for an absentee ballot reject mail-in ballots at much higher rates than states that vote only by mail.

With the USPS’s ongoing slowdown in delivery standards, which have recently made the news, many ballots will probably not reach their intended destinations. Voters in states that vote only by mail mostly return their ballots by using drop boxes, reducing the risk of loss. The Trump campaign has brought a lawsuit to ban drop boxes in Pennsylvania and may file more elsewhere. If these suits are successful, those states’ voters will have a harder time returning their ballots securely.

But the greatest risks of voting by mail are voters’ own mistakes. To minimize this problem, election officials can warn voters that a mistake on their absentee ballot means it won’t be counted — or they can design ballots and instructions using plain language.

Congregating in crowded polling places puts the health of voters and 1 million poll workers on the line. But voting by mail has different risks. Minimizing both types of risks will take election officials and voters working together.

Charles Stewart III is the Kenan Sahin distinguished professor of political science at MIT, director of the MIT Election Data and Science Lab, co-director of the Caltech/MIT Voting Technology Project and co-director of the Stanford-MIT Healthy Elections Project.