During a conversation with friends I realised I needed a better understanding of seasonal flu viruses, and how or if Swine Flu and Bird Flu are seasonal, whether there are other seasonal influenza viruses, and what protection the flu vaccine provides, as, we were all just guessing answers to these points!
I couldn't readily find all of the answers online in one place, so I've pieced together information from various sources and created some comparison tables covering the more widely discussed swine, avian and seasonal influenza viruses.
This post is by no means supposed to be a comprehensive review of each virus, but an overview to give readers a better understanding of seasonal flu, the viruses that cause it, and the flu vaccine.
Feel free to comment with any questions or corrections, all pertinent information is referenced, the majority of which came from various pages of the World Health Organisation’s and CDC's website, alongside data from several recent publications.
I’m discussing types A and B influenza virus owing to their relevance in human infection and vaccination programs, however types C and D also exist, the former of which also infects humans.
Type B influenza notably only infects humans (alongside seals), whereas type A viruses have a range of hosts, some of which include pigs and birds or both.
The ability of influenza A strains (or “subtypes”) to infect other animals aside from humans, known as “zoonosis”, is an important characteristic to understand.
The nomenclature used to differentiate between different strains of influenza A relate to the subtype of two cell surface proteins on the virus. To date there are 18 different subtypes of hemaggluttinin (HA), and 11 subtypes of neuraminidase (NA) used to classify virus subtypes.
Protection is provided through vaccination with antigenic portions of a virus to provoke an immune response. No live virus is used, but the immune response generated against antigenic portions used in a vaccine can cause flu like symptoms, such as a high temperature, due to the body’s natural response when encountering what the immune system believes is a virus. This first encounter by the immune system results in an immune response being mounted and the establishment of immunological “memory”; this takes quite a while, ~10-14 days, which, if encountering a real virus for the first time can result in clinical symptoms and the body not being able to mount a sufficient immune response quickly enough to halt the spread, resulting in the host being overwhelmed. By immunising you create “memory” against the virus, and upon reencounter, an immune response is generated very quickly, taking ~3 days, enabling clearance of the virus often before any symptoms manifest.
You can find out more about how The WHO choose their subtypes for coverage via their webpage, but largely it is based on ongoing studies searching for possible emerging virus subtypes in animals capable of transmitting influenza (mainly birds and pigs), and closely monitoring the health of people working directly with them.
The composition of the vaccine includes coverage for Type A viruses often associated with infections in pigs, termed “Swine Flu” in the media, alongside some Type B influenza virus. Subtypes of virus associated with Bird Flu are not covered in seasonal vaccines. I’m not entirely sure why there is no coverage in the seasonal vaccine(?) but based on risk there are much fewer instances of human infection with Bird Flu and human-to-human transmission has only been recorded anecdotally [8].
Where human cases of Bird Flu (H5N1) have been identified, the illness has been severe, and as such the strain is on a pandemic watchlist as should it develop the capability of easily being transmitted from human to human, it could cause devasting effects. Owing to this threat, vaccines against the most prevalent subtype, H5N1, do exist but are not available in public health programs (at the time of writing).
Table 1. Comparing subtypes, vaccination and transmission
* based on quadrivalent vaccine
References:
1. Steel J., Lowen A.C. (2014) Influenza A Virus Reassortment. In: Compans R., Oldstone M. (eds) Influenza Pathogenesis and Control - Volume I. Current Topics in Microbiology and Immunology, vol 385. Springer, Cham
2. 1918 Pandemic (H1N1 virus). The CDC. Info Sheet. https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html
3. First Global Estimates of 2009 H1N1 Pandemic Mortality Released by CDC-Led Collaboration. CDC Flu Spotlight.June 25, 2012. https://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm
4. WHO recommendations for the viruses to be used in the 2012 Southern Hemisphere Influenza Vaccine: Epidemiology, antigenic and genetic characteristics of influenza A(H1N1)pdm09, A(H3N2) and B influenza viruses collected from February to September 2011. Vaccine. 2012 Oct 5; 30(45): 6461–6471. Published online 2012 Aug 20. doi: 10.1016/j.vaccine.2012.07.089
5. Recommended composition of influenza virus vaccines for use in the 2019-2020 northern hemisphere influenza season. 21 February 2019 (updated on 21 March 2019). The WHO. https://www.who.int/influenza/vaccines/virus/recommendations/2019_20_north/en/
6. Human infection with a seasonal reassortant A(H1N2) influenza virus – Netherlands. Disease outbreak news 23 March 2018. The WHO. https://www.who.int/csr/don/23-march-2018-seasonal-reassortant-ah1n2-netherlands/en/
7. Novel Swine Influenza Virus Subtype H3N1, United States. P. Lekcharoensuk, et al. Emerg Infect Dis. 2006 May; 12(5): 787–794. doi: 10.3201/eid1205.051060
8. Yang Yang, et al. Detecting Human-to-Human Transmission of Avian Influenza A (H5N1).Emerg Infect Dis. 2007 Sep; 13(9): 1348–1353.doi: 10.3201/eid1309.07-0111. PMID: 18252106
9. A Global Perspective on H9N2 Avian Influenza Virus. T. P. Peacock, et al. Viruses. 2019 Jul; 11(7): 620. doi: 10.3390/v11070620
I couldn't readily find all of the answers online in one place, so I've pieced together information from various sources and created some comparison tables covering the more widely discussed swine, avian and seasonal influenza viruses.
This post is by no means supposed to be a comprehensive review of each virus, but an overview to give readers a better understanding of seasonal flu, the viruses that cause it, and the flu vaccine.
Feel free to comment with any questions or corrections, all pertinent information is referenced, the majority of which came from various pages of the World Health Organisation’s and CDC's website, alongside data from several recent publications.
Influenza type A and B virus
I’m discussing types A and B influenza virus owing to their relevance in human infection and vaccination programs, however types C and D also exist, the former of which also infects humans. Type B influenza notably only infects humans (alongside seals), whereas type A viruses have a range of hosts, some of which include pigs and birds or both.
The ability of influenza A strains (or “subtypes”) to infect other animals aside from humans, known as “zoonosis”, is an important characteristic to understand.
There are two key takeaways for zoonotic influenza strains:
- A virus subtype capable of infecting other animals aside from humans, means there is a continual pool of virus circulating among different animal species making total clearance virtually impossible.
- The co-infection of animals that serve as reservoirs of virus can result in co-infection of a single cell with two viruses, which can lead to gene reassortment between viruses, where segments of the viral genome are swapped between each. This evolutionary mechanisms results in the emergence of new virus strains with different properties, including host tropisms (i.e. a virus that previous only infected pigs underwent gene reassortment with another virus that can infect pigs and humans, and is now also able to infect humans as well), and genes giving rise to resistance against antivirals or evasion from host immune systems [1].
Influenza A virus
Influenza type A is of greatest concern for public health due to the points above, which enable rapid virus evolution which can cause devastating effects. Two notable flu pandemics include the 1918 “Spanish Flu” which caused over 50 million deaths [2] and more recently the 2009 pandemic, causing approximately 500,000 deaths [3]. Both viruses responsibly were influenza A strain H1N1.
The nomenclature used to differentiate between different strains of influenza A relate to the subtype of two cell surface proteins on the virus. To date there are 18 different subtypes of hemaggluttinin (HA), and 11 subtypes of neuraminidase (NA) used to classify virus subtypes.
Influenza B virus
To date, there are two known lineages of Influenza B virus [4]:- B/Yamagata/16/88-like
- B/Victoria/2/87-like viruses
Flu vaccine composition: 2019-2020
Each year the World Health Organisation determines the combination of the flu vaccine, which consists of coverage for three or four virus subtypes predicted to be most prevalent and circulating that season.Protection is provided through vaccination with antigenic portions of a virus to provoke an immune response. No live virus is used, but the immune response generated against antigenic portions used in a vaccine can cause flu like symptoms, such as a high temperature, due to the body’s natural response when encountering what the immune system believes is a virus. This first encounter by the immune system results in an immune response being mounted and the establishment of immunological “memory”; this takes quite a while, ~10-14 days, which, if encountering a real virus for the first time can result in clinical symptoms and the body not being able to mount a sufficient immune response quickly enough to halt the spread, resulting in the host being overwhelmed. By immunising you create “memory” against the virus, and upon reencounter, an immune response is generated very quickly, taking ~3 days, enabling clearance of the virus often before any symptoms manifest.
You can find out more about how The WHO choose their subtypes for coverage via their webpage, but largely it is based on ongoing studies searching for possible emerging virus subtypes in animals capable of transmitting influenza (mainly birds and pigs), and closely monitoring the health of people working directly with them.
The composition of the vaccine includes coverage for Type A viruses often associated with infections in pigs, termed “Swine Flu” in the media, alongside some Type B influenza virus. Subtypes of virus associated with Bird Flu are not covered in seasonal vaccines. I’m not entirely sure why there is no coverage in the seasonal vaccine(?) but based on risk there are much fewer instances of human infection with Bird Flu and human-to-human transmission has only been recorded anecdotally [8].
Where human cases of Bird Flu (H5N1) have been identified, the illness has been severe, and as such the strain is on a pandemic watchlist as should it develop the capability of easily being transmitted from human to human, it could cause devasting effects. Owing to this threat, vaccines against the most prevalent subtype, H5N1, do exist but are not available in public health programs (at the time of writing).
Table 1. Comparing subtypes, vaccination and transmission
Common Name
|
Virus type and subtype*
|
Protection included in seasonal flu vaccine* 2019/20 [5]
|
Human-to-human transmission?
|
Swine Flu
|
A(H1N1)
|
Yes
|
Yes
|
A(H3N2)
|
Yes
|
Yes
|
|
A(H1N2)
|
No (but the virus originated from a reassortment of H1N1
and H3N2. [6] so the vaccine should be protective)
|
Not enough public information.
|
|
A(H3N1)
|
No (but the virus originated from a reassortment of H1N1
and H3N2 [7], so the vaccine should be protective)
|
Not enough public information.
|
|
Bird Flu (Avian Flu)
|
A(H5N1)
|
No
|
No but reported anecdotally [8].
|
A(H7N9)
|
No
|
No
|
|
A(H9N2)
|
No
|
No [9]
|
|
Influenza B Virus
|
B/Victoria/2/87 lineage
|
Yes
|
Yes
|
B/Yamagata/16/88 lineage
|
Yes
|
Yes
|
* based on quadrivalent vaccine
General symptoms of infection with influenza virus:
Flu virus infections (generally) begin by inhaling virus particles, and as such there is a commonality in clinical presentation based on the shared primary site of infection. This includes mild upper respiratory infection, resulting in a cough and often fever in response to viral infection. Respiratory tract infections can lead to severe pneumonia, and in some cases acute respiratory distress syndrome (ARDS), shock and even death.Diagnosis:
Laboratory tests are required to identify the virus subtype by the presence of specific surface protein subtypes. This is most commonly performed by PCR.Treatment:
Where available vaccinations, notably for seasonal flu, should be taken by those most at risk. Antiviral drugs given to treat flu include neuraminidase inhibitors, which should be prescribed as soon as possible to prevent or hinder virus replication. Treatments are recommended for a minimum of 5 days but may be extended until a clinical improvement has been achieved.References:
1. Steel J., Lowen A.C. (2014) Influenza A Virus Reassortment. In: Compans R., Oldstone M. (eds) Influenza Pathogenesis and Control - Volume I. Current Topics in Microbiology and Immunology, vol 385. Springer, Cham
2. 1918 Pandemic (H1N1 virus). The CDC. Info Sheet. https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html
3. First Global Estimates of 2009 H1N1 Pandemic Mortality Released by CDC-Led Collaboration. CDC Flu Spotlight.June 25, 2012. https://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm
4. WHO recommendations for the viruses to be used in the 2012 Southern Hemisphere Influenza Vaccine: Epidemiology, antigenic and genetic characteristics of influenza A(H1N1)pdm09, A(H3N2) and B influenza viruses collected from February to September 2011. Vaccine. 2012 Oct 5; 30(45): 6461–6471. Published online 2012 Aug 20. doi: 10.1016/j.vaccine.2012.07.089
5. Recommended composition of influenza virus vaccines for use in the 2019-2020 northern hemisphere influenza season. 21 February 2019 (updated on 21 March 2019). The WHO. https://www.who.int/influenza/vaccines/virus/recommendations/2019_20_north/en/
6. Human infection with a seasonal reassortant A(H1N2) influenza virus – Netherlands. Disease outbreak news 23 March 2018. The WHO. https://www.who.int/csr/don/23-march-2018-seasonal-reassortant-ah1n2-netherlands/en/
7. Novel Swine Influenza Virus Subtype H3N1, United States. P. Lekcharoensuk, et al. Emerg Infect Dis. 2006 May; 12(5): 787–794. doi: 10.3201/eid1205.051060
8. Yang Yang, et al. Detecting Human-to-Human Transmission of Avian Influenza A (H5N1).Emerg Infect Dis. 2007 Sep; 13(9): 1348–1353.doi: 10.3201/eid1309.07-0111. PMID: 18252106
9. A Global Perspective on H9N2 Avian Influenza Virus. T. P. Peacock, et al. Viruses. 2019 Jul; 11(7): 620. doi: 10.3390/v11070620
Comments
Post a Comment