How sick a person gets from a virus can depend onhow much of the pathogen that person was exposed to and how much virus is replicating in their body — questionsthat are still open for the novel coronavirus.
Why it matters: As people try to balance resuming parts of their daily lives with controlling their risk of COVID-19, understanding the role of viral load could help tailor public health measures and patient care.
<p><strong>Driving the news: </strong>An <a href="https://www.medrxiv.org/content/10.1101/2020.09.22.20199125v1.full.pdf" target="_blank">analysis</a> of 5,000 genomic sequences of the coronavirus from patients found those infected with a now-dominant strain with a specific mutation "had higher loads of virus in their upper respiratory tracts, a potential factor in making the strain spread more effectively," the Washington Post <a href="https://www.washingtonpost.com/health/2020/09/23/houston-coronavirus-mutations/?arc404=true" target="_blank">reports</a>.</p><ul class="ee-ul"><li>Viruses <a href="https://www.axios.com/coronavirus-virus-mutation-3c02a0ce-89c0-4866-a869-bad0e538c390.html" target="_blank">typically mutate</a> as they spread through a population, and the mutation that is accumulating wasn't linked to the virus becoming deadlier, according to the study, which hasn't yetbeen peer-reviewed. </li><li>But the research underscores open questions about COVID-19: How does the amount of virus in someone affect transmission to others, and theseverity of the disease?</li></ul><p><strong>How it works: </strong>Viral dose is how much virus someone is exposed to when they are infected. Viral load is the amount of virus produced in someone's body after they are infected. </p><ul class="ee-ul"><li>A higher infectious dose of a virus and a higher viral load is linked to more severe disease from <a href="https://pubmed.ncbi.nlm.nih.gov/25416753/" target="_blank">influenza</a>, <a href="https://jvi.asm.org/content/80/1/306" target="_blank">poxviruses</a> and other viruses.</li></ul><p><strong>For SARS-CoV-2, </strong>the virus that causes COVID-19, "there's accumulating data on both sides of the equation," Monica Gandhi, a professor of medicine and an infectious disease doctor at UCSF, tells Axios. </p><ul class="ee-ul"><li>In a <a href="https://www.pnas.org/content/117/28/16587" target="_blank">study of Syrian hamsters</a>, for example, those infected with a higher dose of SARS-CoV-2 had worse outcomes than those with smaller amounts, supporting an <a href="https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa644/5848814" target="_blank">earlier study</a> on hamsters checking if "masks" helped prevent transmission, she points out.</li><li><a href="https://www.medpagetoday.com/infectiousdisease/covid19/86734" target="_blank">Outbreak investigations</a> show that where there's universal masking of a population, the severity of disease goes down.</li><li>Some researchers suggest a decline in death rate and the rise in asymptomatic cases in the U.S. this summer — both coming at a time when mask-wearing became more common — indicate reducing the dose of the virus <a href="https://www.nytimes.com/2020/07/27/health/coronavirus-mask-protection.html" target="_blank">may reduce</a> the severity of disease.</li></ul><p><strong>"It's very interesting</strong> that it seems like it's correlating with masking, this lower rate of severe illness," says Gandhi, adding that the hypothesis remains unproven but has some growing evidence behind it.</p><ul class="ee-ul"><li>She co-authored a <a href="https://www.nejm.org/doi/pdf/10.1056/NEJMp2026913?articleTools=true" target="_blank">perspective piece</a> in the <em>NEJM </em>this week that says universal adoption of masks could reduce infections and severity until an effective vaccine is distributed. </li><li>Last week, CDC director Robert Redfield <a href="https://www.axios.com/coronavirus-vaccine-redfield-cdc-masks-30db3e33-dc30-40ed-ad5b-f364a0e544c0.html" target="_blank">suggested</a> face masks are "more guaranteed" to offer protection against the coronavirus than a potential vaccine.</li></ul><p><strong>What's happening: </strong>Evidence is emergingabout the <a href="https://www.thelancet.com/journals/lanres/article/PIIS2213-2600%2820%2930354-4/fulltext" target="_blank">link between viral load</a> and <a href="https://www.thelancet.com/journals/laninf/article/PIIS1473-3099%2820%2930232-2/fulltext" target="_blank">how severe</a> COVID-19 is for a patient.</p><ul class="ee-ul"><li>In a study of more than 3,000 patients in three New York City hospitals, nearly 40% who tested positive for COVID-19 and had a high viral load died in the hospital.</li><li>But the risk of dying was lower — dropping to about 15% — for patients with a low viral load, Michael Satlin, an infectious disease specialist at Weill Cornell Medicine and NewYork-Presbyterian, and colleagues reported last week in the journal <em><a href="https://www.cell.com/cancer-cell/fulltext/S1535-6108%2820%2930481-5" target="_blank">Cancer Cell</a></em><a href="https://www.cell.com/cancer-cell/fulltext/S1535-6108%2820%2930481-5" target="_blank"></a>. </li><li><strong>Of note: </strong>The study was done at the height of the city's outbreak in the spring, and Satlin says that given improvements in caring for patients with COVID-19, he wouldn't expect to see such high mortality rates today. </li><li>And the amount of virus the patients were exposed to or their viral load when symptoms started wasn't known — connecting those dots will require more testing, he adds.</li></ul><p><strong>Yes, but: </strong>Other studies have also found some <a href="https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2769235?guestAccessKey=dccb637d-4f65-443e-82af-abde015966be&utm_source=For_The_Media&utm_medium=referral&utm_campaign=ftm_links&utm_content=tfl&utm_term=080620" target="_blank">people without symptoms</a> can have viral loads similar to those with symptoms. And children, who tend to be spared severe COVID-19 complications, can <a href="https://jamanetwork.com/journals/jamapediatrics/fullarticle/2768952" target="_blank">carry as much</a> or more of the virus in their upper respiratory tract.</p><ul class="ee-ul"><li>"Viral load is a part of the picture, but it's not the full picture," infectious disease specialist Ravina Kullar <a href="https://www.medpagetoday.com/infectiousdisease/covid19/88692" target="_blank">told MedPage Today</a>, adding that disease severity depends on a person's immune system.</li></ul><p><strong>What to watch:</strong> Knowing a patient's viral load could be helpful to providers indetermining how therapies should be directed.</p><ul class="ee-ul"><li>Viral load information is in the PCR tests done to confirm SARS-CoV-2 infection but the tests aren't approved by the FDA for that quantitative information, Satlin says.</li><li>If it could be released to clinicians, "it could be very useful" for determining care for people who are hospitalized, he adds.</li></ul><p><strong>Go deeper: </strong><a href="https://www.axios.com/coronavirus-face-mask-science-e9df446b-e7fb-447b-a980-67a9cf73556a.html" target="_blank">Where the science stands on using face masks against coronavirus</a></p></div>
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