EXHAUSTIVE ATTEMPTS to make safe plans, especially for the holidays, revolve around testing—as do the most perplexing coronavirus conundrums: when, for what reasons and which tests to get. Whatever the drawbacks of any kind of test, almost any result can contribute to figuring out the best path forward.
The holiday recommendation from former Baltimore health commissioner Leana Wen: Quarantine for 14 days and then take a rapid-response test, ideally the day before gathering. Reducing infection risk requires a combination of quarantining, testing and precautions –- which includes handwashing, wearing masks and distancing.
The CDC last week issued new guidance defining “close” contact as 15 minutes of cumulative exposure within six feet in a 24-hour period — in contrast to the previous formula of 15 consecutive minutes.
But there is nothing magic about 15 minutes—or about six feet, writes Ryan Malosh, infectious disease epidemiologist at the University of Michigan School of Public Health. Malosh sees the recent guidance as indication that new evidence has emerged—but mostly as “important recognition of the ease with which this virus can spread.”
Among coronavirus tests, one category is diagnostic tests that assess active infection, using nasal or sputum samples—with two test types: P.C.R. (polymerase chain reaction) tests that detect genetic material, require laboratory processing and give results in anywhere from one day to two weeks; and rapid-response antigen tests that detect viral surface proteins responsible for provoking an immune response, can give results on the spot in 15 minutes and may become available for personal use, similar to pregnancy tests.
(In the other category are blood tests that detect antibodies, which the body produces several days after infection begins and which remain in the body for weeks or longer. Because the strength and duration of coronavirus antibodies are not well understood, a positive test result may not ensure protection—but can provide confirmation of infection late in the illness, in cases of suspected post-infectious syndromes and in epidemiological population studies.)
Debate about the value of the different tests used for diagnosis—P.C.R. tests and quick antigen tests—focuses on risks of false negative and false positive results.
The faster antigen tests, when given in random screening of students and staff members at the University of Arizona, detected only 32% of the positive cases identified by the P.C.R. test—creating a high rate of false negatives for the quick test that can encourage an unwarranted sense of security. And compared to the P.C.R. test, the antigen test also produced more false positives— which can be especially dangerous if people who in fact do not have COVID seek treatment or are put in quarantine alongside those with active infections.
Some experts argue, however, that rapid antigen tests are better at pinpointing specifically those individuals most likely to be contagious, and that the greater sensitivity of P.C.R. tests leads to positive results when there is too little virus in the body to spread it to other people. When the Arizona subjects included only those with symptoms, results were similar for both kinds of tests.
In addition, further testing of asymptomatic subjects with positive P.C.R. results found that some had no live virus at all, indicating that they were not contagious. But Jennifer Dien Bard, director of the clinical and microbiology and virology lab at the Los Angeles Children’s Hospital, warned about the inability to predict which sufferers would be carrying live virus: “There is no magic number cutoff for infectiousness.”
And about the low 32% detection rate of the antigen test, Dien Bard pointed out, “That’s worse than flipping a coin.” An additional risk if antigen tests become available for personal use: people who test positive will fail to let anyone know they have the virus.
Meanwhile, as wait times for P.C.R. test results are lengthening again as case numbers rise, the previously recommended strategy of getting tested before a trip or event has changed to testing four to five days beforehand and then maintaining quarantine until the results are in. Says former CDC director Thomas Frieden, “What good is testing if the results take four days to come back and infectious people aren’t isolated in the interim?”
Beyond testing, the next hope is a vaccine—with the candidate developed by Pfizer and BioNTech just announced in its final stage of trials; and the possibility of enough doses by the end of the year for high-risk groups: health care workers, older adults, those with risk factors like diabetes and people of color. To date this vaccine has shown 90% effectiveness, compared to around 50% for flu vaccines and 97% for the measles vaccine.
But not until application is made for “emergency use authorization,” expected by late November, will FDA scientists have a chance to examine raw data on safety and effectiveness. Remaining questions include how long the vaccine’s protection will last, and whether it can protect against severe cases or asymptomatic cases. One difficulty with this vaccine is that storage during transport must be kept at minus 94 degrees Fahrenheit.
Clinical trials for another dozen or so leading vaccine candidates are also in late stages—while China has proceeded to vaccinate hundreds of thousands of people around the world with its four candidates. Many criticize this strategy, but others respect Chinese science and believe the experience might yield good information about risk and effectiveness.
Similar to the workings of a vaccine, when someone wearing a mask does become infected, the reduced dose of virus is likely to produce asymptomatic infection—the case on cruise ships, at food processing plants and in jails—conferring immunity without severe infection.
Mask wearing accompanied by social distancing offered the greatest protection in a Harvard study of 104 workers in a Massachusetts supermarket. Although 91% of workers wore masks, only two-thirds said they could consistently practice social distancing—and those in contact with customers were five times more likely to be among the 21% who tested positive.
In the long run, lessons from the coronavirus about testing, vaccine development and protection may help deal with newly emerging infectious diseases (about 40 since the 1970s that have included SARS, MERS, Ebola and Zika) “at a rate that has not been seen before,” the WHO warns.
“People are traveling more frequently and far greater distances, living in more densely populated areas and coming into closer contact with wild animals,” according to a Baylor College of Medicine report. And in northern Denmark, farmed mink may be responsible for almost 400 recent coronavirus infections.
Other factors spurring the increase of emerging diseases include climate change, as warming temperatures allow mosquitoes to move into new regions; antimicrobial resistance; and a decline in vaccine coverage—the reason measles cases have been highest by far during the last decade.
Looking ahead to the cold weather, respiratory viruses thrive in dry, warm indoor conditions—with the statistical peak of most flu seasons coming just after Christmas and New Year’s, writes Joel Achenbach in the Washington Post. And recently experts have linked the national spike in coronavirus infections to household transmission, rather than to superspreader events.
Among the most important lessons from this coronavirus: Wear a mask and maintain social distancing. Also consider saying no to holiday invites this year. And when reason or opportunity presents itself, take a test—any test.
Well-Being Editor Mary Carpenter keeps us updated on Covid-19. To read more of her posts, click here.