Josh Thomas, a self-employed computer programmer in Port Orchard, Wash., was worried about his job prospects after nearly three months in lockdown because of COVID-19. So last May he signed up online for an offer from his credit card company, Barclays Bank, to defer his monthly payments for up to three months.

But three months later, Thomas discovered that his credit score had dropped after checking his credit reports. Those detailed credit histories produced by the big credit bureaus—Experian, Equifax, and TransUnion—are used by companies like FICO and VantageScore to calculate a three-digit credit score. And that score is used by lenders when deciding whether to offer credit cards and auto, home, and student loans to consumers, and what interest rates to charge.