some people with covid 19 do just fine others die and then there's everyone in between so what
determines disease severity yeah most of us know the risk factors by now it's mostly being older
age being male obese diabetic high blood pressure but not everyone with severe covid fits that mold
for example there was the case of the 28 year old female who needed to have a double
lung transplant in chicago but what about the number of viral particles you're exposed to
or how many viral particles that invade your body before the immune system defeats it if
it's able to defeat it the number of viral particles that land on your mucous membranes
when an infected person coughs or sneezes that's the viral dose an infection can start with a few
viral particles however the higher the dose the more likely the immune system
will not clear all of the viral particles and an infection takes hold the viral dose required to
make you sick seems to be very low for covid 19 as it spread through casual contact the cdc changed
its definition of close contact to a cumulative exposure of 15 minutes or more in contact with
an infected person because it appears that even a series of small exposures may also cause disease
we may not know the minimum infectious dose of sars cov2 needed to result in illness or
whether multiple small doses or a single large dose is more likely to lead to serious disease
the amount of virus someone gets into their system it depends not only on the duration that they're
exposed to someone with the virus but also the viral concentration that you're exposed to
in addition to proximity and this is why social distancing and masks are so important
in order to mitigate this very process viral load is the level of replicating virus in the body
and it's measured in copies per milliliter we can determine whether the sars cov2 virus is present
in the human body by collecting samples from the nose the respiratory tract blood or urine when the
sars cov2 virus reaches the respiratory tract and binds to the ac2 receptors on human cells viral
particles enter respiratory cells and reproduce specifically this occurs in type 2 alveoli
the newly made viral particles infect other cells and the cycle continues this cycle can carry on
and exponentially increase the number of viral particles until the immune system has a chance
to tame it and hopefully stop that replication process whatever that number may be this is what
we refer to as the viral load studies have shown that the higher the viral load in nasopharyngeal
swabs or from blood the more severe the disease will be with higher viral loads equating to two
times or higher odds of needing a breathing tube meaning being intubated and putting on
a breathing machine and this is regardless of age other risk factors and severity of illness at the
time of presentation to the hospital quantifying the viral load is a potential tool for physicians
to risk stratify patients a reverse transcription polymerase chain reaction or rt-pcr is used to
detect the styroscopy to infection on a swab of nasal secretions a lab technologist prepares
the swabs for processing and adds the fluid from the swabs and other chemicals to the pcr machine
the pcr machine goes through a series of heating and cooling cycles during which it makes copies of
any viral rna in the sample each time the pcr machine goes through its series of cycles it
doubles the number of rna strands present the pcr machine can report whether the virus was present
and if so how many cycles it had to go through in order to amplify the rna enough so it's detectable
the cycle threshold if no virus is present in that sample the pcr machine can go through innumerable
amplification cycles and the virus will still not be present if low levels of virus are present
it will take more pcr amplification cycles to get enough rna to make those sars cov2 sample
detectable if there are large amounts of sars cov2 in that sample that means there's going to
be fewer amplification cycles required to get an observable sample so fewer amplification
cycles to get a detectable level of rna equals a higher viral load since multiple studies have
demonstrated that higher viral loads equate to a more severe illness it therefore makes sense that
reporting the number of pcr cycles needed to get a detectable level of sars cov2 would be a valuable
tool to predict who might have more severe disease so there was a study of 308 hospitalized patients
who were infected with covid 19 in china they underwent nasopharyngeal swabbing for pcr at
least five times each and it showed viral load peaked on days two through four of admission
and then decreased rapidly those patients who were most severely ill had the highest viral load other
researchers showed that the human immune response to infection is dependent on both viral load and
infection time course they also show differences in this immune response between men and women
and with aging which may explain why covid 19 is more severe with aging and with males having
data on viral load may help researchers better understand the natural course of covid infection
in a literature review 18 studies were identified that discussed the correlation between pcr and
viral load most of the studies showed a strong association between the number of pcr cycles
and viral load that was subsequently measured in samples and there were 15 studies that showed the
association between pcr cycles and the severity of clinical symptoms also a declining viral load in
many samples in a community may also help predict when the pandemic is getting better or worse in
that specific community but we have to be cautious though because there are many factors that
may influence the quality of the samples in the reliability of the pcr machine data the number of
viral particles on the sample swap may be affected by the stage of the disease how well colonized the
nose is with the virus how the sample was taken and the amount of nasal secretions on the swab
besides the variability in the samples being used there are no standardizations across
pcr platforms so it would be challenging to compare measurements from one lab to another
for these reasons using the data from rt pcr cycles to stratify risk for either an
individual or community may be more useful when evaluating trends compared to making individual
medical decisions at any single point in time so from a clinical standpoint checking pcr levels of
patients who come to the hospital it's not all that useful at least not at this point in time