Human Serology & Flu

CDC Human Serology Efforts to Improve Seasonal Influenza Vaccines and Prepare Against Future Influenza Pandemics

Serology is the scientific study of blood serum to look at the antibody and other responses of the immune system to vaccination or infections with pathogens, like flu viruses. CDC Influenza Division's human serology work supports the following objectives:

• Improving assessment and selection of candidate vaccine viruses (CVVs) for use in producing seasonal flu vaccines.
• Increasing scientific understanding of the characteristics of the immune response following flu vaccination that are associated with immune protection (also known as "correlates of protection.")
• Determining population immunity against circulating human seasonal influenza viruses.
• Identifying asymptomatic or mild flu infections and improving the assessment of flu disease burden.
• Determining population immunity against zoonotic (animal-origin) influenza viruses with pandemic potential.
• Improving the assessment and selection of CVVs for use in producing pre-pandemic flu vaccines

Human serology work to improve assessment and selection of CVVs for producing seasonal flu vaccines

CDC's Influenza Division now uses human serology data to improve the selection of CVVs for use in producing seasonal flu vaccines. Influenza viruses are always changing. Type A and B influenza viruses circulate around the world and cause seasonal flu epidemics. All flu viruses undergo small genetic changes over time, and these genetic changes can give an influenza virus the ability to evade existing immunity a person acquired from prior flu infection or vaccination.

This is one reason the composition of flu vaccines must be reviewed and updated and new vaccines must be produced prior to each flu season.

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 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). VCMs occur in February to recommend the composition of Northern Hemisphere flu vaccines and in September to recommend the composition of Southern Hemisphere flu vaccines.

Seasonal flu vaccines are designed to include CVVs that protect against the three or four (depending on vaccine) major influenza virus groups (known as types, subtypes, and lineages) that often co-circulate during flu seasons. This 2024-2025 season, all available flu vaccines in the United States are trivalent (three-component) flu vaccines containing an influenza A(H1N1)pdm09 component, an influenza A(H3N2) component, and a flu B/Victoria lineage component. Research indicates if the influenza virus representing each vaccine component needs to be updated with a new CVV for each season.

Human serology data contribute to the selection of CVVs for flu vaccines.

Human serology data are now included in the overall body of data that CDC collects to inform WHO's Northern and Southern Hemisphere flu vaccine composition recommendations. To obtain this serology data, CDC works with partner organizations to collect blood samples from people who received flu vaccines. The blood samples are collected during the flu season, and this work begins shortly after flu vaccines become available. Blood samples are collected from people prior to vaccination and then again about 3-4 weeks post-vaccination to allow time for their bodies to develop antibodies. Antibodies are proteins made by a person's immune system in response to infections or vaccination that can help protect against future infections. Following vaccination, a person's antibody counts will gradually increase and typically peak at around 3-4 weeks post-vaccination. Antibody levels from vaccination wane over time, generally over the course of several months post-vaccination, but this does not mean that they drop to zero.

For purposes of serologic testing and analysis, laboratory scientists will take the blood samples collected from people and prepare sera from them. Serum (the singular form of sera) is a fluid component of blood from which cells and blood clotting elements are removed. Sera contain antibodies and are used in serology tests, also known as antibody tests. These tests are conducted on a person's blood serum to look for the presence of antibodies. CDC collects blood samples from people of all age groups and from different geographic areas before and after flu vaccination. CDC uses this blood to prepare sera for analysis of antibodies against circulating influenza viruses. Laboratory tests, including the hemagglutination inhibition assay and microneutralization assay, are commonly used for serology testing.

The goal is to measure how well the antibodies elicited from flu vaccination can recognize and neutralize the influenza viruses in circulation. If the antibodies produced from vaccination effectively neutralize circulating flu viruses, it is likely that the current flu vaccine's composition is suitable for protecting people during that flu season and does not need to be updated. In contrast, if the antibodies produced from vaccination do not effectively neutralize currently circulating influenza viruses, it is likely that one or more of the components of the flu vaccine will need to be updated. All of CDC's human serology data collection and analysis is completed prior to the WHO VCM in order to contribute to the flu vaccine composition recommendations.

How human serology data differs from the ferret sera used traditionally

Scientists have traditionally used sera produced by ferrets inoculated with specific influenza viruses to assess how similar (match) or different (mismatch) the CVVs chosen for use in flu vaccines are to the influenza viruses currently 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 influenza viruses.

The human immune response to flu vaccination can be different from that of ferrets. Ferrets used for this kind of testing are naïve to flu, meaning that they have never been infected with a influenza virus or been vaccinated against flu. In contrast, humans - particularly older children and adults - often have had previous flu vaccinations or influenza virus infections. These prior flu virus infections or vaccinations are known to affect the way the human immune system responds to future flu vaccinations or flu virus infections. For example, sometimes only antibodies contained in human sera (as opposed to ferret sera) can detect the affect of genetic changes between influenza viruses. People may also respond differently to flu vaccination based on their age, exposure history to influenza, geographic location, immune status, and other host factors.

As a result, CDC collects and tests sera from vaccinated people of different ages (from infants 6 months of age to adults 65 years and older) and from different geographic locations, both in the United States and around the world during each flu season. The collection and use of human serology data are critical to ensure that the CVVs used in seasonal flu vaccines can provide sufficient protection against circulating influenza viruses each flu season.

Timing of human serology data collection

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 and prior to the WHO VCM. The WHO VCM takes place in February for the Northern Hemisphere and in September for the Southern Hemisphere.

Human serology work to better understand the correlates of protection

One goal of CDC Influenza Division's human serology work is to increase scientific understanding of the characteristics of the immune response following vaccination that are associated with immune protection, also known as "correlates of protection." As part of this work, CDC Influenza Division conducts immunogenicity studies to compare the protective benefits provided by different flu vaccine platforms. For example, studies are conducted comparing standard egg-based flu vaccines to cell-based, recombinant, adjuvanted, and high dose flu vaccines that are currently licensed for use in the United States. CDC also compares the human antibody response across different age groups and in different populations, including those with different immune status (e.g., children, older adults, health care workers, pregnant people, and people with certain underlying medical conditions). These data help CDC determine the most effective flu vaccines for specific age groups or populations.

Researching the causes of breakthrough infections

Human serology is also useful for studying flu "breakthrough infections," which are flu infections that occur in people who have been vaccinated. CDC researchers want to better understand the immunological factors that are associated with breakthrough infections. For example, scientists have traditionally measured the amount of antibodies that a person produces following flu vaccination by a laboratory test called the hemagglutination inhibition test (HI test). The HI test is used to determine how well antibodies developed against one influenza virus prevent the binding of another influenza virus to red blood cells. This can help CDC researchers determine how antigenically similar the two viruses are to one another. The HI test is often used as a surrogate of the neutralization test.

Certain thresholds of HI antibody titers have been shown to be associated with a reduction in the risk of flu infection (immune protection). A titer is a laboratory unit of measurement for a specific amount of antibody in a serum. HI titers are often used to evaluate how well a flu vaccine works. However, recent data show that other markers (for example, antibodies to the stalk region of the influenza virus's hemagglutinin (HA) surface proteins and antibodies to the virus's neuraminidase (NA) surface proteins) may also contribute to immune protection.

To better understand the causes of breakthrough infections, CDC scientists analyze the sera collected from people with breakthrough flu infections using a variety of additional laboratory tests. In addition to microneutralization assays, which measure antibodies that can neutralize an influenza virus's ability to cause infection, there are assays to measure antibodies that work against the influenza virus's neuraminidase (NA) surface protein; assays to measure antibodies that target the stalk region of an influenza virus's hemagglutinin (HA) surface protein; and other assays to identify additional immunological markers that correlate with protective immunity.

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.

Learning why vaccine effectiveness varies by flu season

Another area of human serology research involves understanding why flu vaccine effectiveness varies from season to season and differs between influenzaflu virus types and subtypes. Many factors can impact the effectiveness of flu vaccines in the population. One factor is the similarity (or match) between the characteristics of the flu vaccine and the influenza viruses that are circulating and causing illness. For example, many flu vaccines are still produced in chicken eggs, and the egg-based vaccine manufacturing production process can introduce genetic changes in vaccine viruses that allow them to grow better in chicken eggs. These changes can alter the antigenic properties of vaccine viruses, making them less similar to the influenza viruses in circulation, and potentially reduce the flu vaccine's effectiveness against circulating influenza viruses. In addition, complex host immunological factors also can affect the protective immunity people receive from flu vaccination. For example, "immune priming" is a concept where the benefit a person gets from vaccination (or the robustness of a person's immune response to vaccination) may be affected by previous flu vaccinations or influenza virus infections that occurred earlier in that person's life.

Determining population immunity against circulating human seasonal influenza viruses

CDC's Influenza Division also conducts human serology work to measure population immunity against circulating seasonal influenza viruses. This is done using seroprevalence surveys. A seroprevalence survey is an investigation that uses serology tests (also known as antibody tests) to estimate the percentage of people in a population that have antibodies against a virus of interest, such as seasonal influenza viruses circulating among people. The presence of these antibodies in blood is an indicator of previous infection or vaccination. Seroprevalence surveys can tell us how many people in a specific population have existing antibodies against a particular influenza virus. The percentage of people in a population who have antibodies to a particular virus is called "seroprevalence."

Seroprevalence surveys can also help determine what proportion of people in a population lack antibodies to emerging influenza 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.

How assessments of population immunity help with improving flu vaccines

Seroprevalence surveys can help CDC determine if the population surveyed is vulnerable to emerging influenza viruses. This enables flu experts to determine whether the flu vaccine should be updated to protect against these influenza viruses for the upcoming flu season. Flu experts choose vaccine viruses that provide broad protection against a range of influenza viruses of the same flu type, subtype, or lineage.

Using serology to identify past evidence of flu virus infection.

Seroprevalence surveys are useful for identifying past mild or asymptomatic influenza 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 influenza 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.

Determining human population immunity against zoonotic flu viruses with pandemic potential

Just as CDC uses seroprevalence studies to assess population immunity against seasonal influenza viruses, CDC also uses the same seroprevalence surveys to assess population immunity against zoonotic (animal-origin) influenza viruses with pandemic potential. For example, CDC can test the human antibodies collected by seroprevalence surveys to determine if people of a particular age or from a particular geographic area are vulnerable to infection with influenza viruses that originate in animals. Examples of influenza viruses that originate in animals include avian influenza (bird flu) viruses and swine influenza viruses (which are called "variant" influenza viruses when they infect people). When an influenza virus from animals infects people for the first time, it is called a novel influenza virus. The risk of a novel influenza virus causing a pandemic can be high if the virus is highly transmissible (i.e., spreads easily among people) and the human population has low or no pre-existing immunity against the virus. CDC closely monitors the emergence of novel influenza viruses and assesses their pandemic risk using what's known as the Influenza Risk Assessment Tool (IRAT). Population immunity is a critical factor in assessing the pandemic potential of emerging novel influenza viruses.

Improving the assessment and selection of CVVs for use in producing pre-pandemic flu vaccines

By using human serology data to determine people's susceptibility to zoonotic influenza viruses with pandemic potential, CDC can prioritize the development of pre-pandemic vaccines that protect against these viruses. For pandemic preparedness, the U.S. government has established pandemic vaccine stockpiles. As new influenza viruses with pandemic potential emerge, CDC scientists evaluate whether stockpiled pre-pandemic vaccines can offer protection against them. This work helps determine whether new pre-pandemic vaccines are needed and informs the selection of CVVs to protect against novel influenza viruses with pandemic potential.