Difference Between Serology And Immunology
Ray Kowalewski
Rubella-virus specific IgM antibodies (rubella IgM) develop shortly after primary infection or vaccination. Rubella-virus specific IgG antibodies (rubella IgG) become detectable by day four after rash onset in rubella infections and reach peak levels in the following one to two weeks. While rubella IgM persists for up to three months, rubella IgG can last a lifetime.
Detection of specific IgM antibodies in a serum sample collected within the first few days after rash onset can provide presumptive evidence of a recent rubella virus infection. The optimum time-point for collection of serum is five days after the onset of symptoms (fever and rash) when >90% of cases will be IgM positive.
On the day of rash onset only about 50% of cases are IgM positive. Therefore, if serum that collected less than five days after onset is negative, a second sample would be necessary to confirm or rule out rubella. When CDC receives serum samples to test for clinically suspected rubella, CDC also determines the anti-rubella IgM or IgG status for each sample to aid in case classification. The interpretation of rubella laboratory results must always take into account relevant clinical and epidemiological data.
Since no assay is 100% specific, serology testing of non-rubella cases occasionally produces false positive IgM results. In the United States where endemic circulation of wild-type rubella virus has been eliminated and the chance of contracting the disease domestically is low, most suspected cases are not rubella.
The measurement of rubella IgG antibody avidity can be used to distinguish between recent exposure to rubella and more distant rubella exposure. Antibody avidity (the overall strength of binding between the antigen and antibody) increases with time; this is known as maturation of the immune response. As the immune response matures, low avidity antibodies are replaced with high avidity antibodies.
Detection of rubella IgM by enzyme immunoassay (EIA) is used to confirm suspected cases of acute rubella infection and congenital rubella syndrome (CRS). IgM is typically detectable between 4 and 30 days after rash onset in acute rubella infection and for up to six months after birth in CRS cases. A person may also be rubella IgM positive after recent vaccination.
Detection of rubella IgG by EIA is used to assess immunity to rubella. EIA can reliably detect rubella IgG concentrations greater than 10 IU/ml, which is the cutoff for defining rubella immunity used in the United States. Rubella IgG serology testing should be used for assessing rubella immunity before, during, and after pregnancy.
Routine IgM screening of pregnant people is not recommended. Rubella IgM testing in asymptomatic, unexposed pregnant people is inappropriate because false positive results can occur, which can be misleading. Rubella IgM testing in pregnant people should be limited to suspected rubella cases. However, providers sometimes inappropriately order IgM tests to assess rubella immunity.
CRS was eliminated from the United States in 2004, but cases can still be imported by pregnant people who contract rubella in an endemic country. In rare cases, CRS can occur in the United States when susceptible pregnant people are exposed to confirmed rubella cases.
CRS cases can be diagnosed in newborns and young infants using detection of rubella IgM. Suspected cases should be tested as close to birth as possible and again at 1 month of age if the initial IgM test is negative. If paired sera are to be collected, the second sample should be collected 14 to 21 days after the acute specimen was collected. At 3 months of age, approximately 50% of cases would still have detectable rubella IgM in their serum. Additionally, the presence of rubella IgG in an infant after the decline of maternal antibodies (9 months of age) and the absence of vaccination or exposure to rubella will confirm CRS.
If IgM testing is inappropriately performed and an IgM positive result is obtained for an asymptomatic pregnant patient with no history of rubella exposure, the IgM positive result should be confirmed by retesting in a different laboratory with a different EIA test format. Preferably, testing should be performed by an IgM capture EIA because IgM capture assays are more specific than indirect IgM EIA assays.
Testing for rubella IgG avidity can be used to distinguish between recent exposure to rubella and a more distant exposure. Avidity testing is most useful in early pregnancy to help rule out a rubella infection in the first trimester, when the risk of congenital defects due to rubella is highest. It is not as useful in late pregnancy because avidity will be high by the third trimester if infection occurred in the first trimester.
Rubella reverse transcription polymerase chain reaction (RT-PCR) assay can also be used to confirm acute rubella infection, but its utility is limited because of the narrow window when the virus can be detected in clinical samples in acquired rubella cases. In respiratory samples, rubella RNA is typically detectable from two days before rash onset to four days after rash onset. In elimination settings, the positive predictive value of rubella RT-PCR in a respiratory sample from an asymptomatic, IgM-positive pregnant patient is low.
Currently used rubella RT-PCR assays have not been validated for testing amniotic fluid or other fetal specimens and should not be used for confirmation of suspected CRS. Although amniotic fluid has been considered for detection of rubella RNA by RT-PCR when fetal infection is suspected, the sensitivity of this method is less than 86%, particularly before the 21st week of gestation. RT-PCR assays on throat swabs, nasopharyngeal swabs, and urine specimens from a neonate are used for confirmation of suspected CRS cases.
The selection of CVVs for producing seasonal flu vaccines is made months prior to the start of the flu season in the Southern and Northern Hemispheres. Twice a year, the World Health Organization (WHO) organizes a consultation with the seven WHO Collaborating Centers, Essential Regulatory Laboratories, and representatives from key national laboratories and academia to review available data and make recommendations on the composition of flu vaccines for the coming season. These WHO consultations are called vaccine composition meetings (VCM). VCM occurs in February to recommend the composition of Northern Hemisphere flu vaccines and in September to recommend the composition of Southern Hemisphere flu vaccines. For more information, visit Selecting Viruses for the Seasonal Flu Vaccine.
Seasonal flu vaccines are designed to include CVVs that protect against the three or four (depending on vaccine) major flu virus groups (known as types, subtypes, and lineages) that often co-circulate during flu seasons. All available flu vaccines in the United States are quadrivalent (four-component) flu vaccines this season. Quadrivalent flu vaccines contain a flu A(H1N1)pdm09 component, a flu A(H3N2) component, a flu B/Yamagata lineage component, and a flu B/Victoria lineage component. Trivalent (three-component) flu vaccines contain only one flu type B virus. Therefore, three or four candidate vaccine viruses (CVVs) are chosen for use in flu vaccine production. Research indicates if the flu virus representing each vaccine component needs to be updated with a new CVV.
Scientists have traditionally used sera produced by ferrets inoculated with specific flu viruses to assess the similarity (match) or differences (mismatch) between the CVVs chosen for use in flu vaccines and the flu viruses that are in circulation. By testing antibodies found in human sera after flu vaccination, scientists can better determine whether the human immune response to the vaccine will sufficiently target and neutralize circulating flu viruses.
CDC collects human serology data year-round to inform the recommendations for the composition of seasonal flu vaccines. For WHO vaccine recommendations, there is only a short timeframe each season after flu vaccines become available during which CDC can collect and analyze the serology data prior to the WHO VCM. The WHO VCM takes place in February for the Northern Hemisphere and in September for the Southern Hemisphere. Therefore, CDC must collect and analyze human serology data twice a year in advance of these meetings to support the flu vaccine composition recommendation for the coming season.
Data gathered using these tests have the potential to provide critical information that will improve understanding of the underlying immunological factors contributing to vaccine breakthrough infections. This information can in turn be used to inform the most effective vaccination strategies.
Alternatively, such surveys can help determine what proportion of people in the population lack antibodies to emerging flu viruses, and therefore, are susceptible to infection with these viruses. CDC collaborates with partner organizations to collect blood samples from thousands of people of all age groups. Sera are collected from people living in diverse geographic locations across the United States so that the populations sampled are representative of the U.S. population.
Seroprevalence surveys can help CDC determine if the population surveyed is vulnerable to emerging flu viruses. This enables flu experts to determine whether the flu vaccine should be updated to protect against these flu viruses for the upcoming flu season. Flu experts choose vaccine viruses that provide broad protection against a range of flu viruses of the same flu type, subtype, or lineage. More information is available at types of flu viruses.
Seroprevalence surveys are useful for identifying past mild or asymptomatic flu virus infections (i.e., infections without symptoms of illness) in people who are no longer shedding virus. Such infections are often missed by traditional diagnostic tests. Identifying these types of infections is especially helpful during investigations of outbreaks caused by novel viruses. In addition, by capturing both asymptomatic and symptomatic infections, human serology data can help health officials better estimate the true burden of flu disease.
c80f0f1006