Editor's Note: (Megan Ranney MD, MPH, is an associate professor of emergency medicine; co-founder of GetUsPPE; and a CNN medical analyst. Follow her @meganranney. The opinions expressed in this commentary are her own. View more opinion on CNN.)
(CNN) Recently, a friend called me to ask for my advice.
She has done her best to follow public health guidance -- like masking in public, avoiding big social gatherings and trying to see friends outdoors instead of indoors -- in the lead up to Thanksgiving. She recognizes that she has had some potential exposure to Covid-19, as her kids have been at college, and she has been going to work. Still, she was really hoping to see her extended family for the holiday.
Their plan was to get tested for Covid-19 on Monday, to make sure that she and her kids were negative, and then drive to her parents' house a few hours away to share some time together Thursday.
She wanted to know: would I give this plan, to test and then join a large family gathering, my blessing?
My friend is not alone in asking that question. Commercial labs and sites offering asymptomatic testing are being overwhelmed by Americans seeking to reassure themselves that they're safe to celebrate. They are exhausted by being alone and want just one normal day.
Unfortunately, I had to tell my friend that a single negative test is not sufficient for a safe Thanksgiving.
Now, I'm encouraged that my friend, like most of us, understand that there are two major factors that affect transmission of the virus:
•Whether or not someone at the event is currently infected with Covid-19. This depends on both local prevalence of the virus and how many people are in attendance.
•The dose of the virus you're exposed to, if there's someone sick. This is determined by ventilation; time spent at an event; distance from that person; and use of personal protective equipment like masks.
Americans are also understanding that Thanksgiving is a potentially deadly mix: right now, nearly the entire country is a "hot zone," and the day itself is an environment that by definition encourages transmission of the virus -- indoors, around a table, without masks. According to MyCOVIDRisk.app, created by me and a colleague at the Brown-Lifespan Center for Digital Health, the current risk of infection from a 10-person indoor gathering without a mask, for two hours, is "high" or "very high" (so, 0.5%-5%+) anywhere you go in the country. If even 1% of the 50 million people traveling for Thanksgiving are infected or catch the infection from someone they spend time with, we will be causing 500,000 new infections in a single day -- which is double our current infection rate.
And trying to reduce the risk of the first variable -- whether or not there's someone infectious, at the event -- is of course critically important.
But testing alone is not enough, and should not make people feel that they can ignore CDC's or health experts' recommendations for a safe Thanksgiving.
First, a test just represents a moment in time. Just like you can have a negative pregnancy test today, but already actually be pregnant, similarly -- unless you've been quarantining for the last 14 days -- you could have a negative test today, but still be contagious on Thanksgiving. It takes days after an infection for the test to turn positive.
A report from New Zealand illustrates the danger of this test-then-travel strategy. A traveler had a negative PCR test, and then got on an airplane. Unfortunately, the negative test was obtained just before he turned positive. He was at peak infectivity during the 8-hour flight, and was responsible for seven infections.
Second, a test can be a "false negative," especially if you're asymptomatic. The accuracy of the tests is not perfect.
We have seen the sad consequences of this a "false negative" in our own government. The Rose Garden super-spreader event occurred because the Trump White House thought that if asymptomatic people tested negative on a rapid antigen test, they could forgo other public health prevention measures like masks, distancing, and ventilation. In the weeks following the event, at least 35 people, including President Donald Trump, tested positive.
Finally, the consequences of a negative test is highest when we're mixing households. There have been ample reports of how easily a virus can spread through a weekend gathering, whether a family vacation or a funeral. The risk of a mistake is, therefore, amplified on a weekend like Thanksgiving.
Now, don't get me wrong: testing matters. Our country desperately needs access to tests to diagnose people with symptoms. We also desperately need access to tests to do "surveillance," where we regularly test asymptomatic people and identify pockets of disease before they spread. These are both critically important public health strategies.
But the purpose of testing is not to provide a free pass. It is part of a comprehensive strategy to keep our loved ones safe.
The safest option this year, of course, is to celebrate Thanksgiving only with your immediate household: the people you share a home with every day. You can Zoom or FaceTime with loved ones in your own town or across the country, play virtual games, and chalk this year up to yet another 2020 disappointment.
The next lowest risk, for those who can't abide not seeing their loved ones, is to move the gathering outdoors. If you are going to try this option, though, you have to do it right. Make sure that you're 6+ feet apart from anyone not in your household. Wear a mask whenever you can. Be sure to not touch the same platters or serving utensils. Minimize the time together. And don't get cozy after a couple drinks!
And lastly, if you have a child coming home from college, or if you yourself have been out and about, the best option is to quarantine -- which means, to not be in the same space as anyone else in the household -- for 14 days, and then have a negative Covid-19 test. (We know that approximately 40% of infections are asymptomatic, so the mere fact that we are symptom-free at the end of the 14 days is likely insufficient.)
At the end of the day, I told my friend, a single negative test is better than nothing, but it is not enough. It does not prove that you aren't infectious today, and it certainly doesn't prove that you're not infectious tomorrow. As the health director of San Francisco, Dr. Grant Colfax, has said: "Please do not use testing to determine whether you can travel or not ... A negative test cannot be an excuse to put yourself or others at risk."
For those who wonder: my friend heeded my advice. She's now planning to do a simple, outdoors, socially distanced meal instead of unintentionally exposing her loved ones to illness by sharing a house for a weekend.
I am so glad they will be safe. I hope other families will make the safest possible choices, too.