SARS-CoV-2 (COVID-19), viral load and clinical outcomes; lessons learned one year into the pandemic: A systematic review

Table 1 Manuscript evaluating quantitative viral load assay and coronavirus disease 2019 outcomes. Sixty manuscripts meet the inclusion criteria

Ref./country	Number of patients	Age (yr)	Sampled sites	Quantitative viral load reported as Ct values or Log10 copies /mL /RTPCR gene target	Correlation with severity of sepsis	Correlation with mortality	P value	Merits of the study/key points
He et al[1], China	94	Median 47 yr	Nasopharynx	Ct values/; N gene	Not reported	Not reported	NR	Highest viral load at pre-symptomatic stage and infectiousness peaks before symptom onset.
Xu et al[2], China	51	Median 37 yr	Nasopharynx, BAL, Anal swab	Ct values/; ORF1ab and N gene	No	No	> 0.05	The quantitative viral load and infectiousness may be the similar for primary (imported form epicenter) and secondary and tertiary exposed group of patients but decrease rapidly (in 14 d) in tertiary patients.
Lescure et al[3], France	5	Median 46 yr	Nasopharynx, Stool, Plasma	Log10copies/mL; RdRp-IP1 gene, E gene	No	Inadequate sample size	NR	Presymptomatic patients may have a high viral load and be highly infectious.
Liu et al[4], China	76	Median 50 yr	Nasopharynx	Ct values; Gene not reported	Yes	No	< 0.005	Patients with severe COVID-19 have a higher mean viral load (60 times higher) and long shedding period.
To et al[5], China	23	Median 62 yr	Oropharynx	Log10copies/mL/; RdRp gene	Yes	Not reported	0.56	Peak viral load occurs at onset of symptoms and is correlated with increasing age and severity although not statistically significant.
Shen et al[6], China	5	Median 60 yr	Nasopharynx	Ct values; Gene not reported	Yes	No	NR	Patients with severe sepsis and high quantitative viral load benefit from convalescent plasma. The viral load became negative in all 5 patients in 12 d with clinical improvement.
Duan et al[7], China	10	Median 52.5 yr	Nasopharynx	Ct values; ORF1ab and N gene	Yes	No	< 0.001	Resolution of severe sepsis and negative viral load with convalescent plasma infusion.
Chen et al[8], China	48	Median 63 yr	Oropharynx. serum	Ct values; ORF1ab and N gene	Yes	Yes	< 0.001	Serum viremia and viral load associated with severity and poor prognosis. High RNAaemia is associated with elevated IL-6 levels.
Pan et al[9], China	82	Not reported	Oropharynx. Sputum, Stool	Log10copies/mL; N gene	Yes	Yes	NR	Viral load is high on presentation. Stool samples may turn positive later in the disease.
Cao et al[10], China	199	Median 58 yr	Oropharynx	Log10copies/mL; N and E gene	Not reported	No	NR	Lopinavir-Ritonavir did not aid with clinical improvement, reduce mortality or reduce the viral loads.
Wang et al[11], China	237	Median 65 yr	Oropharynx, Sputum	Log10copies/mL; Gene not reported	Not reported	Not reported	NR	Remdesivir group does not decrease viral load compared to control group, however it may have faster time to clinical improvement.
Zou et al[12], China	18	Median 59 yr	Nasopharynx, Oropharynx	Ct values; ORF1b	Not reported	Not reported	NR	High viral load begins in the presymptomatic period and may suggest high infectivity.
Wang et al[13], China	23	Median 56 yr	Nasopharynx, Oropharynx, sputum, fecal, urine, plasma	Ct values; RdRp and N gene	Yes	None	< 0.001	High viral load and shedding from multiple tissues occurs for a prolonged period in severe cases. Feces remains positive for a prolonged time.
Wölfel et al[14], Germany	9	Not reported	Oropharynx, Sputum, stool, serum, urine	Log10copies/mL; RdRp and E gene	No	No	NR	High viral load begins in the presymptomatic period and may continue beyond 10 d after symptoms ensue suggest high infectivity. No positivity in stool, urine or serum. All cases were with mild symptoms.
Zheng et al[15], China	96	Median 55 yr	Nasopharynx, Oropharynx, sputum, fecal, urine, plasma	Ct values and Log10copies/mL; ORF1ab	Yes	Not reported	0.03	High respiratory viral load associated with disease severity and serum positivity and stool shedding occurs later and persists for a longer period.
Baggio et al[16], Swiss	352 adults, 53 children	Mean 36.5 yr	Nasopharynx	Log10copies/mL; ORF1ab and E gene	Not reported	Not reported	NR	Children and adults can have same variation of viral loads, but risk of transmission and lower susceptibility in children may have other contributing factors.
Shi et al[17], China	114	Median 43.5yr	Oropharynx, serum	Log10copies/mL; N gene	Yes	Not reported	< 0.001	High viral loads associated with severe sepsis in female patients.
Clementi et al[18], Italy	200	Mean 64 yr	Nasopharynx	Ct values; ORF1ab and E gene	Yes	Not reported	0.08	Higher viral loads associated with older age group and severity of sepsis.
Kwon et al[19], Korea	31	Mean 50 yr	Nasopharynx	Ct values; RdRp and N gene	Yes	None	0.093	High viral loads correlated with elevated cytokine profile and severity of sepsis.
Yu et al[20], China	92	Mean 55 yr	Sputum	Ct values/N and ORF1b	Yes	No	0.017	Higher baseline sputum viral load on admission is associated with severe disease.
Liu et al[21], China	31	Median 58 yr	Nasopharynx, sputum	Ct values; ORF1ab and N gene	Not reported	Not reported	NR	Viral load is higher in deep sputum samples and have a higher shedding and transmission capacity.
Zhou et al[22], China	31	Median 41 yr	Nasopharynx	Ct values; ORF1ab and N gene	No	No	NR	Asymptomatic patients have high viral loads and continue viral shedding and transmission.
Cheung et al[23],Hong Kong	59	Median 58.5 yr	Stool	Log10copies/mL; Gene not reported	No	No	= 0.019	Stool viral loads are higher in patients with diarrhea and may persist after negative respiratory specimens.
Azzi et al[24], Italy	25	Mean 61.5 yr	Saliva	Ct values; 5’UTR	Yes	Not reported	= 0.04	High salivary viral loads may be associated with severe disease and may persist after the negative respiratory specimens. High viral load associated with high serum LDH suggestive of tissue damage.
Chen et al[25], China	22	Median 36.5 yr	Saliva, feces, Oropharynx	Ct values; ORF1ab and N gene	No	No	NR	Sputum and stool viral load remains positive after pharyngeal samples turn negative. Indicating the infectivity may persist after negative pharyngeal samples.
Huang et al[26], China	16	Median 59.5 yr	Nasopharynx, sputum, tracheal aspirates, fecal, urine, plasma	Ct values; N gene	Yes	No	< 0.01	In severe cases higher viral load is demonstrated in deep sputum and tracheal aspirates compared to upper respiratory tract specimens.
Pujadas et al[27], United States	1145	Mean 64.6 yr	Nasopharynx	Log10copies/mL; RdRp and N gene	Yes	Yes	= 0.003	High viral load is an independent predictor of mortality.
Arons et al[28], United States	57	Mean 75 yr	Nasopharynx, Oropharynx	Ct values; N1 and N2	No	Not reported	NR	High viral loads demonstrated in presymptomatic, asymptomatic cases, favoring high transmissibility in close knit nursing home population.
Huang et al[29], China	308	Median 63 yr	Nasopharynx, Oropharynx	Ct values; ORF1ab	Yes	Yes	< 0.001	High viral load associated with critical disease and mortality. Sputum samples have higher viral loads.
Magleby et al[30], United States	678	Median 69 yr	Nasopharynx,	Ct values; ORF1b and E gene	Yes	Yes	< 0.001	High viral load is an independent risk factor for severe sepsis, intubation and death.
Park et al[31], Korea	46	Median 26 yr	Nasopharynx, Oropharynx, sputum , Stool	Ct values; RdRp, N and E gene	No	No	NR	High fecal viral load and shedding, follows and persists after respiratory symptoms resolve for up to 50 d.
Yu et al[32], China	76	Median 40 yr	Nasopharynx, Oropharynx, sputum, urine, plasma	Ct values; ORF1b and N gene	Yes	None	< 0.001	Digital droplet PCR is superior for patients with high suspicion but negative RTPCR. High viral load correlated with risk for progression and disease activity.
Blot et al[33], France	14	Median 67 yr	Broncho-alveolar fluid	Log10copies/mL; RdRp	Yes	Not reported	= 0.013	Higher viral load associated with worse sepsis related organ failure (SOFA) scores.
Kim et al[34], Korea	13	Median 30 yr	Nasopharynx	Ct values; RdRp and E gene	No	No	NR	Patient with mild or asymptomatic infections are infectious before symptoms appear and 14 d of isolation may be sufficient in asymptomatic carriers.
Argyropoulos et al[35], United States	205	Median 60 yr	Nasopharynx	Log10copies/mL; RdRp and N gene	Decreased	Decreased	< 0.001	Study shows inverse correlation of high viral load with duration, severity of sepsis and no correlation with survival.
Xu et al[36], China	85	Median 56 yr	Nasopharynx, Oropharynx, serum	Ct values; ORF1b and N gene	Yes	Yes	< 0.001	Detection of high serum viral load in the serum increases the severity of organ damage, sepsis and mortality.
Veyer et al[37], France	58	Median 55.1 yr	Plasma	Log10copies/mL; ORF1b and N gene	Yes	Yes	= 0.036	Detection of high Viral load in the serum increases the severity of sepsis and mortality.
Lin et al[38], China	217	Median 50 yr	Nasopharynx, Oropharynx, anal	Ct values; ORF1b and N gene	Yes	No	= 0.006	Anal viral load remains positive longer and is correlated with severity of sepsis and ICU admission.
Wang et al[39], China	275	Median 49 yr	Oropharynx	Ct values; ORF1b and N gene	No	No	= 0.824	Similar viral loads between severe and mild cases, no correlation of viral load to ICU admission, severity or mortality.
Kimball et al[40], United States	23	Mean 80.7 yr	Nasopharynx, Oropharynx	Ct values; N1, N2 genes	No	No	= 0.3	High viral loads in unrecognized asymptomatic and presymptomatic patients may contribute to infectiousness and transmission.
Schwierzeck et al[41] ,Germany	12	Not reported	Nasopharynx	Ct values; E and RdRp genes	Yes	No	= 0.007	High viral load, 200 times greater in symptomatic patients compared to asymptomatic patients.
Xia et al[42], China	10	Mean 56.5 yr	Nasopharynx	Ct values; ORF1ab and N gene	Yes	No	NR	Higher viral load associated with severe symptoms and increased neutrophil/lymphocyte ratio.
Huang et al[43], China	41	Median 49 yr	Nasopharynx, Oropharynx, sputum, BAL	Ct values; 5’UTR	No	No	No	Patients with high viral load with RNAeamia had severe infection, elevated cytokine levels, and mortality but not statistically significant.
Hagman et al[44], Sweden	167	Median 63	Nasopharynx, Oropharynx, sputum, Blood	Ct values; E, RdRp, ORF1 genes	Yes	Yes	P < 0.05	Viral RNAemia on admission was associated with eight fold increased risk of in hospital death.
Prebensen et al[45], Norway	123	Median 64	Nasopharynx, Oropharynx, sputum, Blood	Ct values for respiratory specimens; Log10copies/mL for plasma samples; E gene	Yes	Yes	< 0.001	Higher viral loads associated with ICU admission and death.
Hasanoglu et al[46], Turkey	60	Mean 32	Nasopharynx, Oropharynx, sputum, urine, Blood, rectal	Ct values; RdRp gene	Decreased	Decreased	= 0.0141	Viral loads in younger asymptomatic patients were significantly higher compared to elderly, symptomatic patients.
Kawasuji et al[47], Japan	28	Median 45	Nasopharynx	Log10copies/mL; N gene	No	No	= 0.015	High admission nasopharyngeal viral load associated with increased risk of transmission.
Bermejo-Martin et al[48], Spain	250	Median 66	Nasopharynx, Oropharynx, sputum, urine, Blood, rectal	Log10copies/mL; N gene	Yes	Yes	< 0.001	Increased serum viral load associated with increased severity, mortality and dysregulated host response.
Shlomai et al[49], Israel	170	Median 62	Nasopharynx	Ct values; N gene	Yes	Yes	< 0.0001	Increased hypoxemia, severity and eight fold increase in mortality.
Ra et al[50], Korea	213	Median 25	Nasopharynx	CT value; E, N, RdRp gene	No	No	None	Comparable viral load in asymptomatic and symptomatic patients, asymptomatic patients contribute to ongoing transmission.
Faico-Filho et al[51], Brazil	875	Median 48	Nasopharynx	Ct value; N gene	Yes	Yes	< 0.0001	Admission nasopharyngeal viral load was independently associated with increased mortality.
Chen et al[52], China	52	Median 62	Blood, oropharynx	Log10copies/mL; ORF1ab	Yes	Yes	< 0.001	Increased RNAemia associated with severity, markers of inflammation and mortality.
Fajnzylber et al[53], United States	88	Median 57	Nasopharynx, Oropharynx, sputum, Blood	Log10copies/mL; N gene	Yes	Yes	= 0.009	Increased viremia associated with severity, progression and mortality.
Zhou et al[54], China	195	Median 66	Oropharynx	Ct value; N gene, ORF1ab	Yes	Yes	< 0.005	High viral load associated with multi organ failure and death.
Maltezou et al[55], Greece	1122	Mean 46	Nasopharynx, Oropharynx	CT value; E, RdRp gene	Yes	Yes	< 0.05	High viral load correlated with intubation and in hospital mortality.
Bitker et al[56], France	129	Median 69	Nasopharynx, Oropharynx, sputum	Ct value; ORF1ab	Yes	Yes	< 0.05	High viral load associated with increased mortality.
Carrasquer et al[57], Spain	169	Median 67	Nasopharynx	Ct value; E, N gene, ORF1ab	No	No	= 0.029	High viral load statistically not associated with in hospital mortality.
de la Calle et al[58], Spain	455	Mean 64	Nasopharynx	Ct value; N gene	Yes	Yes	= 0.022	High viral load associated with respiratory failure, and 30 d mortality.
Bryan et al[59], United States	109	Mean 65	Nasopharynx	Ct value; N gene	Yes	Yes	= 0.01	The high nasopharyngeal viral load on admission was independently associated with greater mortality.
Choudhuri et al[60], United States	1044	Mean 65	Nasopharynx	Ct value; ORF1ab	No	Yes	< 0.001	High viral load is an independent predictor of increased mortality.

Data on country of origin, age, number of patients, sample sites, real time reverse transcriptase polymerase chain reaction targets, correlation with sepsis and mortality and key conclusions. NR: Not reported; ORF: Open reading frame; E: Envelope; N: Nucleocapsid; 5’UTR: 5 prime untranslated; RdRp: RNA dependent RNA polymerase; Ct: Cycle threshold; SOFA: Sequential organ failure assessment; ICU: Intensive care unit; COVID-19: Coronavirus disease 2019.