What is viral load

According to the World Health Organization's (WHO's) literature regarding criteria for releasing COVID-19 patients from isolation, the viral load in a patient "peaks within the first week of infection" in the upper respiratory tract, following which there is a gradual lowering over time. However, viral load in the lower respiratory tract and in the faeces has been seen to peak in the second week of illness.

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A study done by researchers in Italy, a country where the new coronavirus infection spread quickly between February and May, found that patients who were tested for COVID-19 in the month of May had lower viral loads than those who were tested a month earlier. This was in line with theories about viral loads in the past, and it also suggested that the lockdown measures that were put in place to control the spread of the disease may have lowered people's exposure to the virus. However, there wasn't any substantial evidence to prove this claim.

Another study in Switzerland took samples from 4,172 people infected by COVID-19 between the months of February and April 2020, and found that people had high viral loads in the early states of the infection, which declined gradually and was primarily characterised by inflammation in the later stages of the disease. The findings were helpful in devising the right treatment strategy for COVID-19 patients as the lowering of the viral load and the predominant symptom could be treated with anti-inflammatory drugs.

Another study, performed in Italy during the early days of the COVID-19 pandemic there, looked at more than 5,000 people but found no difference in the viral loads between people with symptoms or those without.

A study in China took throat swabs from 94 patients also found no difference in the viral loads between people with mild symptoms or those with severe symptoms.

Contrary to the above-mentioned findings, however, a survey of patients at a hospital in New York City showed that people with higher viral loads were less likely to be hospitalised. The study, published in the American Journal of Pathology in July 2020, looked at a batch of 205 patients to study the association of viral loads with the onset of symptoms.

Interestingly, the study suggested that the hospitalised patients had lower viral loads as compared to those who weren't, even after taking into account various factors such as age, sex, race, BMI (body mass index) and comorbidities. However, the researchers found that higher viral load was associated with a shorter duration of symptoms in all patients as well as a shorter stay in the hospital.

The most striking part of the study was that less symptomatic patients had a higher chance of viral shedding, that is, a higher likelihood of infecting other people.

Viral load in a patient is measured with the help of different tests, such as RT-PCR tests, branched DNA or bDNA tests, as well as nucleic acid sequence-based amplification—or NASBA—tests. All the three tests have different methods to calculate the volume of the virus in the body, hence the use of one out of the three is better to gauge more consistent results.

Read more: Viral load test

The amount or volume of viral load in a patient can also be helpful in determining other factors, such as the reproductive number of the virus, or R0 (R-naught), although there are several parameters to be kept in mind in making assumptions about how the virus is spreading or by how much.

Much of the research carried out by the scientific community in previous years has been to understand the spread of HIV/AIDS in human populations, and it is calculations like these that have allowed health workers to be able to combat the disease much more effectively than in the past. 

Similar efforts are ongoing in the battle against COVID-19, as comparisons with the earlier SARS-CoV virus that caused the SARS outbreak of 2002-03 was critical in understanding the nature of the newer, SARS-CoV-2 virus and how it behaved.

The pattern of the spread of COVID-19, much like previous outbreaks of infectious diseases in human history, can be quite difficult to analyse, and it is here that understanding viral load in patients is of critical importance. It not only helps to understand some patterns of the spread of the disease, but also to find out who is more likely to fall sick, become infected, or infect others. 

It also helps understand the role of patients with mild or no symptoms in spreading the infection further than previously thought, and makes a case for the use of masks, face covers and other preventive measures to limit the spread of infection.