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Is SARS-CoV-2 Contagious?


covid-19

SARS-CoV-2, which causes COVID-19, is contagious; however, health experts are still are still uncertain about the exact mode of transmission. Prior to early May 2021, the CDC reported that SARS-CoV-2 was primarily spread person to person through respiratory droplets and often occurred when an infected individual sneezed or coughed near another person. This meant that transmission could occur between individuals spaced more than six feet apart and the virus could linger in the air even after a person had left the area.1

In early May 2021, the CDC updated information regarding SARS-CoV-2 transmission and acknowledged that airborne transmission of the virus was also a primary mode of transmission.2 This acknowledgement, however, occurred nearly three months after leading experts in industrial health, public health, and medicine had sent a letter to the CDC and White House officials requesting that airborne, or inhalation, transmission of the SARS-CoV-2 virus be promptly addressed and emphasized the need to ensure that frontline workers were provided with appropriate respirators and ventilation strategies to reduce the risk of infection and control the spread of the virus. These experts also noted that France, Germany, and Austria had recently mandated N95 filtering facepiece respirators (FFRs) for all frontline workers to reduce virus transmission.3

During a press conference referencing the letter held in mid-February 2021, Dr. Donald Milton, MD, PhD, stated:

"Government officials must fully recognize inhalation exposure as a major way COVID-19 spreads and take immediate action to control and limit this exposure," and "For months the scientific evidence has been clear: Aerosol transmissions are a major way this virus spreads." [4

The CDC also reports that SARS-CoV-2 transmission can occur when a person touches a contaminated object and then touches their mouth, nose, or eyes.5

There is also evidence that the virus can be spread through the fecal-oral route, which supports the recommendation for frequent handwashing. Infected feces can contaminate food, surfaces, and hands and potentially lead to illness.6 In China, health officials have begun using anal swabs to test for SARS-CoV-2 and report their accuracy to be superior to nasal and throat swabs. Anal swabbing was initiated after research noted that traces of the virus can remain longer in the anus than in the respiratory tract.7

Additionally, a study published November 19, 2020 in The Lancet Microbe reported that the SARS-CoV-2 virus is most contagious in the first five days after the onset of symptoms. This study also found that there was no difference in the viral loads among asymptomatic or symptomatic SARS-CoV-2-positive individuals, and research indicates that people without symptoms clear the virus quicker and are therefore less contagious.8

SARS-CoV-2 Testing

Importantly, persons who test positive for the SARS-CoV-2 virus may not go on to develop COVID-19 illness. On average, individuals exposed to SARS-CoV-2 will begin to develop COVID-19 symptoms within five days. Most people who develop symptoms will do so within 12 days. Rarely, symptoms may begin within two days or after 14 days.9

Persons that do go on to develop COVID-19 illness may experience mild to moderate symptoms and are generally no longer considered infectious 10 days after the onset of initial symptoms. Persons who develop severe COVID-19 illness are not likely to be infectious after 20 days post-symptom onset; however, persons who are severely immunocompromised may be capable of shedding the virus for longer than 20 days, and as long as 144 days after testing positive for SARS-CoV-2.10

The reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) test used to identify individuals infected with the SARS-CoV-2 virus uses a nasal swab to collect RNA from deep within the nasal cavity and is then reverse transcribed into DNA and amplified through cycles. Results from a PCR test are reported as a simple “yes” or “no” answer to the question of whether someone is infected.11

However, the validity of COVID-19 testing has continually been questioned by some health experts. There are reports that the PCR testing for SARS-CoV-2, the virus associated with COVID-19 illness, is too sensitive and adjustments are needed to distinguish people who have insignificant amounts of harmless viral material in their system versus people who are clinically infected with live virus. The most significant concern is that the number of detection cycles for the test is so high that it reports a positive result for people who have infectious live virus as well as a positive result for people who have only a few genetic fragments left over from a past infection and no longer pose a risk to others.12 13

A New York Times review published on August 29, 2020 reported on three sets of testing data that included cycle thresholds (CT) values compiled by officials in Massachusetts, New York and Nevada. The review found that “up to 90 percent of people testing positive barely carried any virus” and experts stated that tests using high CT values may be detecting not only live virus, but also genetic fragments; “leftovers from an infection that pose no particular risk” for contagiousness. The review reported that most tests used a CT value of 40, and noted the CDC’s acknowledgment that samples with a CT value above 33 cycles were unlikely to detect live virus.14

According to the U.S. Food and Drug Administration (FDA), cycle threshold ranges used to determine who is positive are set by commercial manufacturers and laboratories.15

A positive RT-qPCR test does not confirm whether a person is currently ill or will become ill in the future, whether they are infectious or will become infectious, whether they are recovered or recovering from COVID-19, or whether the RT-qPCR test identified a viral fragment from another coronavirus infection in the past. The RT-qPCR test is only capable of reporting that a person has come into contact with coronavirus RNA.16

Carl Heneghan, the director of the Centre for Evidence Based Medicine at the University of Oxford and editor of BMJ Evidence-Based Medicine has expressed concerns regarding the use of PCR testing to confirm a case of COVID-19 and noted:17

“In any other disease we would have a clearly defined specification that would usually involve signs, symptoms, and a test result… We are moving into a biotech world where the norms of clinical reasoning are going out of the window. A PCR test does not equal COVID-19; it should not, but in some definitions it does.”

A systematic review published in December 2020 in Clinical Infectious Diseases focused on COVID-19 viral cultures from PCR samples and assessed their infectious potential. A key finding from this review noted that the data suggests that complete live viruses are necessary for transmission of COVID-19, and not fragments that may be identified by PCR tests utilizing a high CT value. The review also estimated that the recovery of live virus from specimens with a CT value greater than 35 was only 8.3 percent, with five studies in the review reporting no growth in specimens with a CT value ranging from 24 to 35. The review also found that infectious potential declined after day 8, including cases with ongoing high viral loads.18

The CDC also reports that a person who has recovered from COVID-19 may have low levels of virus in their bodies for up to three months after diagnosis and may test positive, even though they are not spreading COVID-19.19

Addressing the controversial CT value aspect of COVID-19 PCR tests, on January 13, 2021, WHO issued a medical product alert and stated that persons who interpret results for specimens tested using PCR methodology should be aware that "careful interpretation of weak positive results is needed."20

A specimen may be considered weak if a high number of cycles are performed prior to virus detection. This may also mean that the person who provided the specimen might not be infectious.21 22 WHO is advising that a positive PCR test that is not consistent with the clinical presentation of COVID-19 should be confirmed through retesting of a new specimen.23

The CDC has revised testing guidelines for SARS-CoV-2 (COVID-19) multiple times and as of September 30 2021 advised persons with symptoms of COVID-19 to get tested and stated that COVID-19 vaccination would not affect test results. Also noted was testing to screen asymptomatic persons when community risk or transmission levels are high, such as testing in the workplace and before travel.24 The CDC also recommends use of antigen testing when confirmatory testing is required. This guidance also recommends isolation individuals with positive test results, and interviewing and notification of close contacts for purposes of quarantine.25

Relating to negative test results, the CDC recommends that the unvaccinated continue to test and quarantine when exposed to SARS-CoV-2.26 As of Oct 4, 2021, the CDC has recommended that fully vaccinated individuals with a potential or known SARS-CoV-2 exposure get tested within 3 to 5 days and wear a mask while in public for at least 14 days or until negative test results are received. Isolation is only recommended in fully vaccinated persons who have symptoms of COVID-19, or who receive positive COVID-19 test results.27

In May 2021, the CDC issued new guidance to laboratories and recommended reducing the RT-PCR CT value to 28 when testing persons previously vaccinated with the COVID-19 vaccine. This guidance was issued in response to reports of breakthrough cases of COVID-19 in fully vaccinated individuals.28 29

Like all testing, SARS-CoV-2 testing is not 100 percent accurate and false-positive and false-negative results can occur. False results are generally attributed to human error in processing, specimen contamination, or to the test’s sensitivity and specificity in accurately detecting the virus.30

Isolation and Quarantine after SARS-CoV-2 Virus Exposure

Since the declaration of the COVID-19 pandemic the CDC has recommended isolation and quarantine measures to limit the spread of SARS-CoV-2 infections. CDC quarantine guidelines have also been revised several times during the course of the declared pandemic.

Quarantine recommendations are intended for individuals who may have been exposed to the virus through close contact to a person with COVID-19. Close contact is defined as being within six feet of an infected person for at least 15 minutes or more in a 24 hour period. Isolation is recommended for infected individuals and in simple terms means to stay away from others when you are ill.31

Guidance published on March 12, 2021 by the CDC updated quarantine guidelines and stated that persons in close contact to a COVID-19 positive person did not need to quarantine, if they have tested positive for SARS-CoV-2 within the previous three months, had recovered, and were asymptomatic. Additionally, it stated that persons who were considered fully vaccinated and who had received the vaccines in the past three months were not required to quarantine as long as they were asymptomatic. Persons in close contact with an infected individual were advised to stay at home for 14 days, and to isolate themselves while ill.32

As of September 18, 2021, the CDC states that individuals with close contact to someone who is positive for SARS-CoV-2 should quarantine for 14 days. Fully vaccinated individuals are not required to quarantine after exposure unless symptomatic; however, testing within 3 to 5 days after exposure is recommended, along with masking in indoor spaces for 14 days or until a negative test result was confirmed. The CDC also states that persons who has had a positive viral test in the previous 90 days, recovered from illness and does not have COVID-19 symptoms does not need to quarantine should they have close contact with an individual with COVID-19. Masking for at least two weeks while in public and monitoring of symptoms are still recommended for this population.33

Immunity after SARS-CoV-2 infection

Researchers report that more than 95 percent of people who recover from SARS-CoV-2 infection had durable immunity to the virus for at least eight months.34 According to a study conducted by researchers at the Washington University School of Medicine in St. Louis, persons who recover from COVID-19 illness, including asymptomatic and mild cases, continue to have lasting immunity upon recovery. The study researchers have speculated that immunity following COVID-19 infection will endure long-term.35

A preprint study posted on medRxiv conducted by the Cleveland Clinic Health System involving 52,238 employees found that "Not one of the 1,359 previously infected subjects who remained unvaccinated had a [Covid-19 infection over the duration of the study” and vaccination did not reduce the risk. Study authors concluded that persons previously infected with SARS-CoV-2 were unlikely to benefit from COVID-19 vaccination.36

An August 25, 2021 retrospective study of Israel’s second largest HMO, yet to undergo peer review, compared 673,676 vaccinated individuals who had not been previously infected by SARS-CoV-2, 62,883 unvaccinated individuals and 42,099 previously infected individuals with a single vaccine dose. The study found that natural immunity “confers longer lasting and stronger protection against infection, symptomatic disease, and hospitalization caused by the Delta variant.”37

IMPORTANT NOTE: NVIC encourages you to become fully informed about covid-19 and the covid-19 vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.

« Return to Vaccines & Diseases Table of Contents

Updated October 20, 2021

References

1 U.S. Centers for Disease Control and Prevention. How COVID-19 Spreads. In: COVID-19. Oct. 28, 2020.

2 U.S. Centers for Disease Control and Prevention. How COVID-19 Spreads. In: COVID-19. May. 13, 2021.

3 Soucheray S. Experts push CDC, White House to address COVID-19 aerosol spread. CIDRAP News Feb. 17, 2021.

4 Soucheray S. Experts push CDC, White House to address COVID-19 aerosol spread. CIDRAP News Feb. 17, 2021.

5 U.S. Centers for Disease Control and Prevention. How COVID-19 Spreads. In: COVID-19. May 13, 2021.

6 Soucheray S. Studies show COVID-19 virus likely has multiple infection routes. CIDRAP News. Feb. 19, 2020. 

7 Parpia R. Anal Swabs Used to Test for SARS-CoV-2 Virus in China. The Vaccine Reaction Feb. 14, 2021.

8 Van Beusekom, M. COVID-19 most contagious in first 5 days of illness, study finds. CIDRAP News Nov. 20, 2020.

9 WebMD. Coronavirus Incubation period. Aug. 11, 2021.

10 U.S. Centers for Disease Control and Prevention. Ending Isolation and Precautions for People with COVID-19: Interim Guidance. In: COVID-19. Sept. 14, 2021.

11 Mandavilli A. Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be. The New York Times Aug. 29, 2020.

12 Lenthang M. Experts: US COVID-19 positivity rate high due to ‘too sensitive’ tests. Daily Mail Aug. 30, 2020.

13 Tom MR., Mina MJ. To Interpret the SARS-CoV-2 Test, Consider the Cycle Threshold Value. Clin Infect Dis. November 2020; 71(16):2252-2254.

14 Mandavilli A. Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be. The New York Times Aug. 29, 2020.

15 Mandavilli A. Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be. The New York Times Aug. 29, 2020.

16 Cáceres B. Coronavirus Cases Plummet When PCR Tests Are Adjusted The Vaccine Reaction Sept. 29, 2020.

17 Mahase E. Covid-19: the problems with case counting. BMJ Sept. 3, 2020; 370:m3374.

18 Jefferson T, Spencer EA, Brassey J, et al. Viral cultures for COVID-19 infectious potential assessment – a systematic review. Clinical Infectious Diseases December 2020; ciaa1764.

19 U.S. Centers for Disease Control and Prevention. Interim Guidance on Ending Isolation and Precautions for Adults with COVID-19. In: COVID-19. Sept. 14, 2021.

20 WHO. WHO Information Notice for IVD Users 2020/05 Jan. 13, 2021.

21 Payne D. WHO warns of 'false positives' in COVID tests, says some patients might not be 'truly infected'. Just the News Jan. 22, 2021.

22 Cáceres M. WHO Issues New Guidance for Determining PCR Test Results. The Vaccine Reaction Feb. 21, 2021.

23 Lee M. WHO changes CCP Virus Test Criteria in Attempt to Reduce False Positives The Epoch Times Jan. 23, 2021.

24 U.S. Centers for Disease Control and Prevention. Overview of Testing for SARS-CoV-2 (COVID-19) In: COVID-19. Aug. 2, 2021.

25 U.S. Centers for Disease Control and Prevention. Overview of Testing for SARS-CoV-2 (COVID-19) In: COVID-19. Aug. 2, 2021.

26 U.S. Centers for Disease Control and Prevention. Overview of Testing for SARS-CoV-2 (COVID-19) In: COVID-19. Aug. 2, 2021.

27 U.S. Centers for Disease Control and Prevention. Quarantine and Isolation. In: COVID-19. Oct. 4, 2021.

28 U.S. Centers for Disease Control and Prevention. Information for laboratories COVID-19 vaccine breakthrough case investigation In: COVID-19 Breakthrough Case Investigations and Reporting. Oct. 5, 2021.

29 Payne D. CDC will only sequence post-vaccine COVID cases detected at lower test sensitivities. Just The News May 12, 2021.

30 Hou CY, False positive and false negative coronavirus test results explained The Hill May 7, 2020.

31 U.S. Centers for Disease Control and Prevention. Quarantine and Isolation. In: COVID-19. Oct. 16, 2021.

32 U.S. Centers for Disease Control and Prevention. Quarantine and Isolation. In: COVID-19. Mar. 12, 2021.

33 U.S. Centers for Disease Control and Prevention. Quarantine and Isolation. In: COVID-19. Oct. 4, 2021.

34 National Institutes of Health (NIH) Lasting immunity found after recovery from COVID-19 NIH Research Matters Jan. 26, 2021.

35 Washington University School of Medicine in St. Louis. Good news: Mild COVID-19 induces lasting antibody protection. May 24, 2021.

36 Shrestha NK, Burke PC, Nowacki AS. et al. Necessity of COVID-19 vaccination in previously infected individuals. medRxiv June. 19, 2021.

37 Gazit S, Shlezinger R, Perez G, et al. Comparing SARS-CoV-2 natural immunity to vaccine-induced immunity: reinfections versus breakthrough infections. medRxiv Aug. 25, 2021.


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