The paper we’re demystifying can be found here, if you’d like to follow along.
This article was reviewed by Dr. Emily Bowman and Dr. Kelly Summers, two of our subject matter experts.
Introducing New Diagnostic Criteria For COVID-19
Serologic tests are used to detect the body’s immune response to a viral infection by measuring the number of antibodies that are present in our blood serum. Usually two types of antibodies, IgG and IgM, are measured. Simply put, this blood test identifies the body’s response to a virus rather than detecting the virus itself.
So why are antibodies produced when our body encounters a virus? We know that antibodies, produced by B lymphocytes (B cells) are the result of our adaptive immune response to anything foreign in our body. Antibodies serve two main purposes in defending our immune system by binding to antigens of a pathogen (the bad stuff that makes us sick) to either stop it from entering our cells, or signal white blood cells to destroy the pathogen.
Blood is first collected from an individual and then separated into different components by spinning the blood in an apparatus called a centrifuge, as seen in the picture below. The top part is a clear yellow fluid called serum. Antibodies are present in the serum. The bottom part appears red as it contains red blood cells.
IgM antibodies are quickly generated by our bodies as an initial response to an infection, then IgG antibodies, which can be even better at fighting viruses but take longer to make, are generated. IgM antibodies continue to be produced alongside IgG until the infection is resolved. To learn more about antibodies and their involvement with COVID-19, read this article by Deanna Kim here.
Serologic tests became available in February 2020 to measure IgG and IgM antibodies produced against SARS-Cov-2, the virus responsible for COVID-19. This led to serologic diagnostic criteria being added to the New Coronavirus Pneumonia Prevention and Control Protocol for the novel coronavirus disease 2019 (COVID-19) to help diagnose individuals infected by this virus. This new protocol was released by the National Health Commission of the People’s Republic of China in March 2020.
What are the implications of the new change in diagnostic criteria?
Previously, a study done on nine pregnant women and their newborns presented no indication that mothers could pass on the SARS-CoV-2 virus to their infants. This study did not use the latest COVID-19 serologic diagnostic criteria to diagnose both the mothers and newborns. Recently, a team of scientists applied the new serologic diagnostic criteria to re-evaluate whether applying the new diagnostic criteria would better assess the possibility of SARS-CoV-2 maternal-infant transmissions.
In this study, the research team reviewed clinical records and laboratory results from six women and their infants who had been confirmed positive through multiple tests between February 16 and March 6, 2020, but had not been subjected to the new serologic diagnostic criteria. All six mothers had mild clinical symptoms of COVID-19. Blood samples and throat swabs were obtained from the mothers and newborns at time of birth.
The researchers were specifically looking to see whether the new serologic criteria implemented would make a difference in the determination of mother-infant SARS-CoV-2 transmissions. In order to run the tests, they first collected blood samples from the mothers and newborns and isolated serum, which was then subjected to IgG and IgM antibody testing.
The antibody tests used had high accuracy as the sensitivity and specificity for IgM antibodies were 88.2% and 99.0% respectively, and for IgG antibodies were 97.8% and 97.9%. Quantitative RT-PCR was also conducted on both neonatal serum and throat swabs to detect the genetic material of the SARS-CoV-2 virus. In addition, another test was also done to measure the level of a protein called interleukin-6 (IL-6) that is produced by activated immune cells during infections. IL-6 levels correspond to the level of inflammation in the newborn.
Great care was taken to minimize chances of aerosolization of virus during delivery. All infants were delivered by caesarean in negative pressure isolation rooms with their mothers wearing masks. Negative pressure rooms are an isolation technique used in hospitals to prevent cross-contamination of contagious diseases that may travel airborne from room to room. A caesarean delivery, also known as a C-section, is a method of birthing through an incision in the mother’s abdomen instead of through the vagina. Infants were immediately isolated from their mothers after delivery, and all medical staff involved wore personal protective equipment consisting of protection suits and double masks.
Putting New Serologic Diagnostic Criteria to The Test
The team first had all six infants undergo both 1-minute and 5-minute Apgar scoring, which is a test that provides a brief summary of newborn health. Apgar scores range from 1 through 10, with 1 being in poor health and anything 7 or above being in good health. Researchers found that all six infants had 1-minute Apgar scores between 8 and 9, and 5-minute Apgar scores between 9 and 10. Furthermore, all neonatal (newborn infants) throat swabs and blood samples that underwent RT-PCR testing came back negative for SARS-CoV-2. As of March 8, 2020, none of the infants involved with the study presented symptoms consistent with COVID-19. The researchers then analyzed the IgG and IgM antibody concentration of the six infants in comparison to results of the mothers.
Two infants presented higher than normal IgG and IgM antibody concentrations, yet, these results corresponded similarly to their mothers who also had elevated levels of IgG and IgM antibodies. Three of the infants had normal IgM levels but higher than normal IgG levels; however, those three infants had mothers with higher levels of IgG, and two with elevated IgM levels.
Typically produced by white blood cells at sites of inflammation, IL-6 helps regulate the body’s immune and inflammatory response by warning the immune system against the inflammation source. As mentioned earlier, IL-6 levels correspond to the level of inflammation in the newborn. The researchers noted that all infants had a significant increase of IL-6 levels, suggesting that there may be inflammation present in the infants.
From the results of the serum tests and throat swabs, SARS-CoV-2 was not detected by RT-PCR in all six infants whose mothers were confirmed with COVID-19. However, the researchers noted that COVID-19-specific antibodies were detectable in neonatal blood serum samples, as observed by increased IgG levels in five of the infants. This is most likely due to IgG antibodies being transferred from mother to fetus by means of the placenta during the end of the second trimester and peaking in concentration at time of birth. In a process called naturally acquired passive immunity, the expectant mother will naturally pass some of her IgG antibodies through the placenta to protect her fetus. It is highly likely that the reported presence of virus-specific IgG antibodies in the infants reflects normal passive immunity from the mother. IgM is NOT transferred via placenta due to its bigger molecular structure. Imagine the placenta as a sieve and antibodies as flour; some of the flour will make it through (IgG) but some larger clumps (IgM) will be caught because it’s just too big in size. In some cases, viruses and antibodies may be transferred if the birth is traumatic, where tearing occurs and there is blood exchanged between the mother and baby. On the other hand, IgM antibodies in infants is much more indicative of viral presence, as the body makes IgM antibodies first in response to infections. Interestingly, two of the infants presented higher than normal IgM concentrations. The researchers suggest that the IgM may have been produced by the infant itself if the virus actually did transfer through the placenta. As this is the most likely scenario, it might indicate that there is a possibility of the virus being exchanged between the mother and newborn.
This area of research presents numerous ongoing investigative studies, and this study is one of many attempting to understand if the SARS-CoV-2 virus can cross the placenta. As discussed in an editorial examining the results of this current study and another similar study, the infants involved in the two studies showed increased levels of IgG antibodies, which may possibly suggest that the virus could have crossed the placenta from mother to infant. However, none of them had positive RT-PCR test results, meaning that there is no viral evidence to support the serologic data (antibody test results). The editorial also mentioned that the precision of antibody tests is less reliable compared to other tests such as the molecular diagnostic tests for example, where there are fewer false-positives (meaning the test result incorrectly tells us that a particular attribute is present when it is actually not.) With much uncertainty of the two studies’ results presented in the editorial, the authors mentioned that more definitive evidence is needed in order to come to a conclusion that fetuses could be at risk of infection from mothers infected with SARS-CoV-2.
Conclusions and Limitations
While this study looks promising, it does come with its own set of limitations. Primarily, a significant limitation would be its small sample size and therefore may present an issue with generalizability. Meaning, with such a small number of individuals involved in this experiment, it would be harder to apply the results to a larger population. Researchers have also noted incomplete information, such as the outcome of infants involved in the study, again with regards to the small sample size. In hindsight, this study offers a quantitative analysis that may ultimately lead to more large-scale studies.
Perhaps in the near future, this research may lead to possible antiviral treatments through the understanding of antibodies relating to SARS-CoV-2 in order to combat the current COVID-19 pandemic. Currently, there are no approved specific antiviral treatments for COVID-19. Isolation of antibodies from infants would not be feasible due to their low blood volume. However, a novel treatment is now being tested by using convalescent antibodies isolated from recovered infected adults. By injecting the antibodies from recovered infected adults into severe COVID-19 patients, it may provide passive immunity and help those with weaker immune systems that are unable to generate an effective antibody response against SARS-CoV-2. A downside to this would be that it would most likely require a large amount of these antibodies to be effective. This would however, greatly benefit the elderly and people with pre-existing health conditions, groups which currently hold the highest death rates from COVID-19.
With many similar studies currently ongoing and due to this study’s very small sample size, it is too early to conclude anything regarding the possibility of SARS-CoV-2 virus passing over from mother to infant. Although these initial findings did not demonstrate direct evidence for placental infection with SARS-CoV-2, this research offers a pathway into further discussions and as an update to other ongoing studies in the similar fields of interest. In addition, having a larger study sample population would improve the overall quality of the experimental results and better answer the question of SARS-Cov-2 transmissibility from mother to infant. Even though the results of this study are encouraging, further investigation down the road is needed to fully understand the importance and implications of serological (blood serum) characteristics from infants whose mothers have been affected by COVID-19.