NerdWallet has released its latest comprehensive analysis of how nine U.S. airlines are responding to the COVID-19 pandemic, looking at both the health and safety measures carriers are taking to ensure passenger safety as well as the flexibility each is offering for new and existing bookings. NerdWallet analyzed things like mask enforcement, fully or partially blocked seats, boarding procedures, passenger self-screening, number of changes allowed, flexibility and vouchers, among others. Here are the results based on analysis as of March 7, 2021.

Still one of the most affordable carriers, budget-friendly Spirit Airlines performed the worst among the nine U.S. airlines that NerdWallet analyzed, scoring just a 1.8 (out of 5) overall. Spirit was the only airline to earn a 0 for health policies but tied American Airlines and United Airlines in terms of flexible booking policies, earning a 3.7 out of the possible 5.

United Airlines ranks worst among the Big Four U.S. carriers with a 2.8 out of 5 overall rating as of this month. United scored just a 2 out of 5 for health and safety policies and a 3.7 for flexible booking policies. The airline is permanently eliminating change fees for most economy and premium cabin tickets for travel within the U.S. and between the U.S. and Mexico or the Caribbean but penalties will apply to basic economy tickets starting April 1.

American Airlines ties United with a COVID-19 response rating of 2.8 out of 5. The airline ties for the lowest score for flexible booking policies (3.7 out of 5) and trails only Spirit when it comes to health and safety policies, tying four other carriers with a 2 out of 5.

Hawaiian Airlines scores well for booking flexibility in the time of COVID-19 (4.3 out of 5) but is held back by a poor mark for health and safety policies (2 out of 5) like most airlines that have stopped blocking seats to allow for physical distancing. Beyond enhanced cleaning and touchless experiences, Hawaiian is assisting customers by organizing all of Hawaii's approved trusted traveling partners by departure city.

Ultra-low-cost carrier Frontier Airlines ranks middle of the pack based on NerdWallet's latest analysis, rating 3.2 out of 5 overall. Frontier's 4.3 rating for flexible booking policies is tied for the third-best. The airline will continue to waive change and cancellation fees for travel through March 31, so long as they're made prior to the required check-in time.

Low-cost Southwest Airlines pioneered flexible change and cancellation policies long before the pandemic so it's no surprise that the carrier scored a perfect 5 out of 5 for flexible booking policies. Unfortunately, the carrier stopped blocking middle seats late last year and still allows customers to choose their own seats, eliminating the possibility of back-to-front boarding procedures that minimize crowding in the aisles.

JetBlue ranks third for health policies (and is one of three airlines to earn better than a 2 out of 5) and ties for third for flexible booking policies. The result is a third-place ranking with a COVID-19 response rating of 3.7 out of 5 overall. The airline recently announced the launch of a digital health pass for customers traveling from Boston to Aruba permitting convenient at-home testing and a streamlined arrival process.

With a NerdWallet score of 4.2 out of 5, Alaska Airlines is all alone in second place. The carrier is the only one providing seat blocking in premium economy and has introduced new touch-free options to help reduce the possible spread of COVID-19.

Delta Airlines continues to lead the way in terms of safety in the COVID era, scoring a perfect 5 out of 5 overall. Delta is currently the only U.S. airline still limiting capacity to keep middle seats open (through April 30) to allow for physical distancing on flights. The airline won all the key categories that NerdWallet considered, including health and safety policies, flexible booking options and expanded travel vouchers.