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Prevention and Prophylaxis of SARS-CoV-2 Infection

Last Updated: February 11, 2021

Summary Recommendations
Summary Recommendations
  • The COVID-19 Treatment Guidelines Panel (the Panel) recommends against the use of any drugs for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pre-exposure prophylaxis (PrEP), except in a clinical trial (AIII).
  • The Panel recommends against the use of hydroxychloroquine for SARS-CoV-2 post-exposure prophylaxis (PEP) (AI).
  • The Panel recommends against the use of other drugs for SARS-CoV-2 PEP, except in a clinical trial (AIII).
  • The Panel recommends that health care providers follow recommendations from the Advisory Committee on Immunization Practices when using SARS-CoV-2 vaccines (AI).
Rating of Recommendations: A = Strong; B = Moderate; C = Optional
Rating of Evidence: I = One or more randomized trials without major limitations; IIa = Other randomized trials or subgroup analyses of randomized trials; IIb = Nonrandomized trials or observational cohort studies; III = Expert opinion

General Prevention Measures

Transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is thought to mainly occur through respiratory droplets transmitted from an infectious person to others within six feet of the person. Less commonly, airborne transmission of small droplets and particles of SARS-CoV-2 can occur at distances greater than six feet, and in rare cases, people passing through a room that was previously occupied by an infectious person may become infected. SARS-CoV-2 infection via airborne transmission of small particles tends to occur after prolonged exposure (more than 30 minutes) to an infectious person who is in an enclosed space with poor ventilation.1

The risk of SARS-CoV-2 transmission can be reduced by covering coughs and sneezes and maintaining a distance of at least six feet from others. When consistent distancing is not possible, face coverings may further reduce the spread of infectious droplets from individuals with SARS-CoV-2 infection to others. Frequent handwashing also effectively reduces the risk of infection.2 Health care providers should follow the Centers for Disease Control and Prevention (CDC) recommendations for infection control and appropriate use of personal protective equipment (PPE).3


Currently, no SARS-CoV-2 vaccine has been approved by the Food and Drug Administration (FDA). In December 2020, the FDA issued Emergency Use Authorizations for two mRNA vaccines, BNT162b2 (Pfizer-BioNTech)4 and mRNA-1273 (Moderna).5 BNT162b2 can be administered to individuals aged ≥16 years, whereas mRNA-1273 can be given to individuals aged ≥18 years.

In large, placebo-controlled trials, these vaccines were 94% to 95% efficacious in preventing COVID-19 after participants completed a two-dose series. Cases of COVID-19 were confirmed by the presence of symptoms and a positive result on a nucleic acid amplification test (NAAT).6,7 Both vaccines also showed efficacy against severe COVID-19. Local and systemic adverse events are relatively common with these vaccines, especially after the second dose; most adverse events were mild or moderate in severity (i.e., they did not prevent recipients from engaging in daily activities). There have been a few reports of severe allergic reactions, including some reports of patients who experienced anaphylaxis after receiving a SARS-CoV-2 mRNA vaccine.8 Safety data continue to be collected. Certain populations, such as pregnant and lactating individuals, were not included in the initial vaccine trials. The American College of Obstetricians and Gynecologists has published interim guidance on the use of the SARS-CoV-2 mRNA vaccines in pregnant and lactating people.9

It is not known how long SARS-CoV-2 vaccines’ protective effect will last or whether SARS-CoV-2 vaccines can prevent asymptomatic infection or transmission, whether they will prevent infection by all current or emergent strains of SARS-CoV-2, whether they will be effective in immunocompromised patients, or whether they will work as well in patients that are at high risk for severe COVID-19 as in those who are at low risk. The efficacy and safety of SARS-CoV-2 vaccines have not been established in children, pregnant people, or immunocompromised patients. Clinical trials for other SARS-CoV-2 vaccine candidates are ongoing.

CDC sets the U.S. adult and childhood immunization schedules based on recommendations from the Advisory Committee on Immunization Practices (ACIP). ACIP considers disease epidemiology, burden of disease, vaccine efficacy and effectiveness, vaccine safety, the quality of the available evidence, and potential implementation issues. ACIP also sets priorities regarding who receives vaccines in the event of a shortage. ACIP COVID-19 vaccine recommendations are reviewed by CDC’s Director and, if adopted, are published as official CDC recommendations in the Morbidity and Mortality Weekly Report.10

Pre-Exposure Prophylaxis

  • The COVID-19 Treatment Guidelines Panel (the Panel) recommends against the use of any drugs for SARS-CoV-2 pre-exposure prophylaxis (PrEP), except in a clinical trial (AIII).


At present, there is no known agent that can be administered before exposure to SARS-CoV-2 (i.e., as PrEP) to prevent infection. Clinical trials are investigating several agents, including emtricitabine plus tenofovir alafenamide or tenofovir disoproxil fumarate, hydroxychloroquine, ivermectin, and supplements such as zinc, vitamin C, and vitamin D. Studies of monoclonal antibodies that target SARS-CoV-2 are in development. Please check for the latest information.

Clinical Trial Data

Randomized Controlled Trial of Hydroxychloroquine for SARS-CoV-2 Pre-Exposure Prophylaxis Among Health Care Workers

This randomized, double-blind, placebo-controlled trial was designed to determine whether hydroxychloroquine 600 mg per day reduced the frequency of SARS-CoV-2 infection over an 8-week period in hospital-based health care workers. The primary outcome was incidence of SARS-CoV-2 infection as determined by reverse transcriptase polymerase chain reaction (RT-PCR) assay of nasopharyngeal swabs collected at 4 and 8 weeks or the occurrence of COVID-19 symptoms.11

Study Population
  • Participants included health care workers at two Philadelphia hospitals who worked ≥20 hours per week in a hospital-based unit, had no known history of SARS-CoV-2 infection, and had no COVID-19-like symptoms in the 2 weeks before enrollment. The study enrolled workers in the emergency department and in dedicated COVID-19 treatment units.
  • The study excluded individuals who were allergic to hydroxychloroquine and those with glucose-6-phosphate dehydrogenase deficiency, retinal disease, or substantial cardiac disease.
  • The study was based on the assumption of a 10% infection rate for the planned inclusion of 100 participants per arm.
  • Between April 9 and July 14, 2020, community infection rates declined. At the time of the second interim analysis (when 125 of 132 participants who provided consent were evaluable for the primary endpoint), the Data Safety Monitoring Board recommended early termination of the study for futility.
  • Four participants in each group developed SARS-CoV-2 infection (positivity rate of 6.3% vs. 6.6% in the hydroxychloroquine and placebo groups, respectively; P > 0.99). Across the groups, six individuals developed symptoms of COVID-19, but none required hospitalization.
  • Serologic testing for anti-spike protein immunoglobulin (Ig) M, IgG, and nucleocapsid protein IgG demonstrated more positive results among participants in the hydroxychloroquine group (four participants [7.4%]) than in the placebo group (two participants [3.7%]), although the difference was not statistically significant (P = 0.40).
  • Mild adverse events were more common among participants in the hydroxychloroquine group than in the placebo group (45% vs. 26%; P = 0.04). The greatest difference was the increased frequency of mild diarrhea in the hydroxychloroquine group.
  • The rates of treatment discontinuation were similar in the hydroxychloroquine group (19%) and the placebo group (16%).
  • There were no cardiac events in either arm and no significant difference in the median frequency of changes in QTc between the study arms (P = 0.98).
  • The study was stopped early.
  • Due to the low SARS-CoV-2 infection rate among the participants, the study was underpowered to detect a prophylactic benefit of hydroxychloroquine.
  • The study population was mostly young, healthy health care workers; therefore, the applicability of the study findings to other populations is uncertain.

There was no clinical benefit of administering hydroxychloroquine 600 mg per day for 8 weeks as PrEP among health care workers who were exposed to patients with COVID-19. Compared to placebo, hydroxychloroquine was associated with an increased risk of mostly mild adverse events.

Hydroxychloroquine as Pre-Exposure Prophylaxis for COVID-19 in Health Care Workers: a Randomized Trial (COVID PREP Study)

This was a randomized, double-blind, placebo-controlled clinical trial to evaluate whether hydroxychloroquine 400 mg given once- or twice-weekly for 12 weeks (compared to placebo) can prevent SARS-CoV-2 infection in health care workers at high-risk of exposure. The primary outcome was COVID-19-free survival time. Diagnosis of COVID-19 was defined as having laboratory-confirmed SARS-CoV-2 infection or having cough, shortness of breath, or difficulty breathing or having two or more of the following symptoms: fever, chills, rigors, myalgia, headache, sore throat, or new olfactory and taste disorders. COVID-19-compatible illness was included as a primary outcome even if a SARS-CoV-2 PCR test was not performed or if it was performed and the result was negative.12

Study Population
  • The study participants had to be working in the emergency department, in the intensive care unit, on a dedicated COVID-19 hospital ward, or as a first responder; alternatively, they had to have a job description that included regularly performing aerosol-generating procedures.
  • Participants were recruited via social media platforms. Informed consent was obtained remotely, and the study drug was delivered to the participants by couriers.
  • The study was powered based on an anticipated 10% event rate of new symptomatic infections. The investigators determined that the study needed to enroll 1,050 participants per arm to have 80% power. However, it became apparent before the first interim analysis that the study would not meet the enrollment target. As a result, enrollment was stopped without unblinding. The investigators attributed the marked decline in enrollment to the negative reports related to the safety of hydroxychloroquine, including a warning from the FDA.
  • Among the 1,483 participants who were randomized, baseline characteristics were similar across the study arms.
  • The number of individuals who met the primary endpoint of confirmed or suspected SARS-CoV-2 infection was 39 (7.9%) in the placebo group and 29 (5.9%) in both the once- and twice-weekly hydroxychloroquine groups. Among the 97 participants, only 17 were confirmed to be SARS-CoV-2 PCR positive.
  • Compared to placebo, the hazard ratio for the primary endpoint was 0.72 (95% CI, 0.4–1.16; P = 0.18) for the once-weekly hydroxychloroquine arm and 0.74 (95% CI, 0.46–1.19; P = 0.22) for the twice-weekly hydroxychloroquine arm.
  • There were no significant differences for any of the secondary efficacy endpoints among the three groups.
  • There were significantly more adverse events reported in the once- and twice-weekly hydroxychloroquine arms (31% vs. 36% of participants experienced adverse events; P < 0.001 for both groups) than in the placebo group (21% of participants). The most common side effects were upset stomach and nausea.
  • Drug concentrations were measured in dried whole blood samples from a subset of 180 participants who received hydroxychloroquine. The median hydroxychloroquine concentrations for the twice- and once-weekly hydroxychloroquine groups were 200 ng/mL and 98 ng/mL, respectively; both of these concentrations are substantially below the in vitro half-maximal effective concentration (EC50) of hydroxychloroquine. The investigators noted that the simulations that were used to determine the hydroxychloroquine dose for the study predicted much higher drug concentrations than the observed levels.
  • The study was prematurely halted due to poor enrollment; therefore, the study population was insufficient to detect differences in outcomes among the study arms.
  • The study only assessed the SARS-CoV-2 inhibitory activity of two doses of hydroxychloroquine, neither of which achieved concentrations that exceeded the in vitro EC50 of the drug.
  • Only 17.5% of the participants who met study endpoints had positive SARS-CoV-2 test results; the remainder had compatible symptoms without a confirmatory diagnosis.

Administering hydroxychloroquine 400 mg once- or twice-weekly did not reduce the number of people with documented SARS-CoV-2 infection or symptoms that were compatible with COVID-19 among health care workers who were at a high risk of infection. These findings suggest that hydroxychloroquine was not effective for SARS-CoV-2 PrEP or that the dose used for this indication was suboptimal.

Post-Exposure Prophylaxis

  • The Panel recommends against the use of hydroxychloroquine for SARS-CoV-2 post-exposure prophylaxis (PEP) (AI).
  • The Panel recommends against the use of other drugs for SARS-CoV-2 PEP, except in a clinical trial (AIII).


At present, there are no known agents that have been shown to be efficacious in preventing infection after exposure to SARS-CoV-2 infection (i.e., as PEP). Several randomized controlled trials have evaluated the use of hydroxychloroquine for SARS-CoV-2 PEP.13-15 None of these studies have reported any evidence of efficacy, and all showed an increased risk of adverse events among participants who received hydroxychloroquine compared to controls. A number of agents (e.g., anti-SARS-CoV-2 monoclonal antibodies, hyperimmune gammaglobulin, convalescent plasma, ivermectin, interferons, tenofovir with or without emtricitabine, vitamin D) are currently being investigated for SARS-CoV-2 PEP. Please check for the latest information.

Clinical Trial Data

Both chloroquine and hydroxychloroquine have in vitro activity against severe acute respiratory syndrome-associated coronavirus (SARS-CoV) and SARS-CoV-2.16,17 A small cohort study without a control group suggested that hydroxychloroquine might reduce the risk of SARS-CoV-2 transmission to close contacts.18

Household-Randomized, Double-Blind, Controlled Trial of SARS-CoV-2 Post-Exposure Prophylaxis With Hydroxychloroquine

A household-randomized, double-blind, controlled trial evaluated the use of hydroxychloroquine as PEP to prevent SARS-CoV-2 infection. The study was conducted at seven institutions in the United States between March and August 2020. Participants were recruited using online advertising, social media, and referrals from hospitals, health departments, and those with laboratory-confirmed SARS-CoV-2 infection.13

Households were randomized to receive oral hydroxychloroquine 400 mg once daily for 3 days, followed by hydroxychloroquine 200 mg once daily for an additional 11 days, or oral ascorbic acid 500 mg once daily for 3 days, followed by ascorbic acid 250 mg once daily for 11 days. Mid-turbinate nasal swabs were collected daily during the first 14 days, with the primary endpoint being PCR-confirmed SARS-CoV-2 infection during the first 14 days after enrollment in those who were not infected at baseline.

Study Population
  • Eligible participants had close contact with an infected person, which included household contacts or other close contacts (82%) or health care workers (18%) who cared for an infected person without wearing appropriate PPE. Participants must have come into contact with an index person who had received a diagnosis of SARS-CoV-2 infection within the past 14 days, and high-risk exposure to the index people must have occurred within the previous 96 hours.
  • Enrollment included 829 participants from 671 households; 407 participants (in 337 households) received hydroxychloroquine, and 422 participants (in 334 households) received ascorbic acid.
  • A total of 98 SARS-CoV-2 infections were detected during the first 14 days of follow-up, with an overall cumulative incidence of 14.3% (95% CI, 11.5% to 17%). Fifty-three events occurred in the hydroxychloroquine group, and 45 events occurred in the control group (aHR 1.10; 95% CI, 0.73–1.66; P > 0.20)
  • In preplanned analyses, hazard ratios were not significantly different within subgroups based on type of contact, time between the most recent contact and the first dose of the study drug, duration of contact, number of contacts enrolled within the household, quarantine status, index case symptoms, or number of adults or children in the household.
  • Adverse events that are associated with the use of hydroxychloroquine, including gastrointestinal symptoms and rash, occurred in 112 participants: 66 participants (16.2%) in the hydroxychloroquine group and 46 participants (10.9%) in the control group (P = 0.026).
  • There was an average window of 2 days between the time of the most recent exposure and the time the study drugs were administered, which may have affected the efficacy of hydroxychloroquine if early initiation is important for efficacy.
  • The primary analysis excluded approximately 10% of enrolled people who were shown to be infected at baseline.

In this study, hydroxychloroquine was ineffective when used as PEP for SARS-CoV-2 infection. Participants who received hydroxychloroquine had an expected increased risk of adverse events when compared to those who received ascorbic acid.

Randomized, Double-Blind, Controlled Trial of High-Risk or Moderate-Risk Occupational or Household Exposures

This randomized, double-blind, controlled trial included 821 participants who self-enrolled in the study using an internet-based survey. Participants were randomized to receive either hydroxychloroquine 800 mg given once, followed by hydroxychloroquine 600 mg given 6 to 8 hours later, and then hydroxychloroquine 600 mg given once daily for 4 additional days or placebo. Because enrollment was done online, study drugs were sent by overnight mail, resulting in more than 50% of participants initiating the first dose of their assigned treatment 3 to 4 days after exposure to SARS-CoV-2.15

Study Population
  • Participants had a high or moderate risk of occupational exposure (66% of participants) or household exposure (34% of participants) to SARS-CoV-2.
  • High-risk exposure was defined as being within six feet of an individual with confirmed SARS-CoV-2 infection for more than 10 minutes while not wearing a face mask or eye shield (87.6% of participants). Moderate-risk exposure was defined as the same distance and duration of exposure while wearing a face mask but no eye shield (12.4% of participants).
  • A total of 107 participants developed the primary outcome of symptomatic illness. Illness was confirmed by a positive result on a SARS-CoV-2 molecular test; if testing was not available, participants were considered to have symptomatic illness if they developed a compatible COVID-19-related syndrome based on CDC criteria.
  • Due to limited access to molecular diagnostic testing, SARS-CoV-2 infection was confirmed in only 16 of the 107 participants (15%). There was no statistically significant difference in the incidence of the primary outcome (symptomatic illness) between the hydroxychloroquine group and the placebo group (11.8% vs. 14.3%; P = 0.35).
  • There were more adverse events in the hydroxychloroquine group (mostly nausea, loose stools, and abdominal discomfort), with no serious adverse reactions or cardiac arrhythmias.
  • Initiation of therapy was delayed for at least 3 days after exposure to SARS-CoV-2 in most participants.
  • Only 15% of participants who reached the primary outcome had SARS-CoV-2 infection confirmed by molecular diagnostics.
  • The study population was young (with a median age of 40 years) and consisted of participants who had a relatively low risk of severe COVID-19.

There was no difference in the incidence of observed symptomatic COVID-19 between participants who received hydroxychloroquine 600 mg once daily and those who received placebo. Although hydroxychloroquine 600 mg per day was associated with an increase in the frequency of adverse events, these adverse events were mostly mild.

Cluster-Randomized Trial of High-Risk Exposures in Spain

This open-label, cluster-randomized trial included 2,314 asymptomatic contacts of 672 COVID-19 cases in Spain.14 Participants who were epidemiologically linked to a PCR-positive COVID-19 case were defined as study clusters (called rings). All contacts in a ring were simultaneously cluster-randomized 1:1 to receive usual care (the control arm) or hydroxychloroquine 800 mg once daily for 1 day followed by hydroxychloroquine 400 mg once daily for 6 days (the intervention arm). Participants were informed of their allocated study arm after being randomized to the intervention or control arm and signing a consent form.

The primary outcome was onset of laboratory-confirmed COVID-19, defined as a positive result on a SARS-CoV-2 PCR test and at least one of the following symptoms: fever, cough, difficulty breathing, myalgia, headache, sore throat, new olfactory and taste disorders, or diarrhea. A secondary outcome was onset of SARS-CoV-2 infection, defined as either a positive SARS-CoV-2 PCR test result or the presence of any of the symptoms compatible with COVID-19. An additional secondary outcome was development of serological positivity at Day 14.

Study Population
  • Study participants were health care or nursing home workers (60.3%), household contacts (27.1%), or nursing home residents (12.7%) who were documented to have spent >15 minutes within two meters of a PCR-positive COVID-19 case during the 7 days prior to enrollment.
  • The baseline characteristics of the participants were similar between the two study arms, including comorbidities, number of days of exposure to SARS-CoV-2 before enrollment and randomization, and type of contact.
  • A total of 138 study participants (6.0%) developed PCR-confirmed, symptomatic SARS-CoV-2 infection, with no statistical difference for this outcome between the control and intervention arms (6.2% vs. 5.7%; risk ratio 0.86; 95% CI, 0.52–1.42).
  • There was no statistical difference between the study arms in the incidence of either PCR-confirmed or symptomatically compatible COVID-19, which occurred in 18.2% of participants: 17.8% in the control arm and 18.7% in the intervention arm (risk ratio 1.03; 95% CI, 0.77–1.38).
  • There was no statistical difference between the arms in the rate of positivity for SARS-CoV-2 IgM and/or IgG (8.7% in the control arm vs. 14.3% in the intervention arm; risk ratio 1.57; 95% CI, 0.94–2.62).
  • There were more adverse events among the hydroxychloroquine-treated participants (56.1%) than among the controls (5.9%), although most of the adverse events were mild. Common adverse events included gastrointestinal events, nervous system disorders, myalgia, fatigue, and malaise. No serious adverse events were attributed to the study drug.
  • The study lacked a placebo comparator, which could have had an impact on safety reporting.
  • Data regarding the extent of the exposure to the index cases was limited.
  • For >50% of the study participants, the time from exposure to the index case to randomization was ≥4 days.

The hydroxychloroquine regimen used for PEP in this study did not prevent SARS-CoV-2 infection in healthy individuals who were exposed to a PCR-positive case.

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