50 Fun Facts About Influenza

How long after exposure do influenza symptoms typically begin?

The incubation period runs one to four days, and symptoms usually last two to eight days.

How many types of influenza virus are there?

Types A and B cause seasonal epidemics, C causes mild illness mainly in children, and D is found in cattle and pigs.

Which animals are the primary reservoir of influenza A viruses?

Ducks, geese, shorebirds and gulls carry them; the virus also circulates in pigs and marine mammals.

Which type lives mainly in cattle and pigs and is not known to cause human illness?

It was only discovered in 2011 and is distantly related to type C.

In the subtype name H1N1, what does the H stand for?

The N is neuraminidase; both are proteins in the viral envelope.

Which two influenza A subtypes have circulated together in humans since 1977?

Both have been in seasonal circulation together since 1977.

How many H types have been isolated from wild birds?

Neuraminidase types run 1 to 11 in birds; H17, H18, N10 and N11 have been found only in bats.

Influenza B has no subtypes but two lineages named after which two places?

The Yamagata lineage may have gone extinct around 2020-21 thanks to COVID-19 measures.

Which of the four contributes to seasonal epidemics but has never caused a pandemic?

It contributes to seasonal epidemics but does not reassort with animal strains the way A does.

What kind of genome do influenza viruses have?

Types A and B have eight segments; the segmentation is what allows reassortment.

How many genome segments do influenza A and B viruses have?

They encode ten major proteins; C and D have seven segments.

What is the gradual build-up of mutations that changes a flu virus's antigens called?

It is the main reason flu vaccines have to be updated every year.

What is antigenic shift?

When an animal strain reassorts with a human one, a novel pandemic-capable strain can emerge.

Which animals are potential 'mixing vessels', carrying both bird and human flu receptors?

Reassortment in such hosts can produce strains capable of human-to-human spread.

Which cell-surface molecule do influenza HA proteins bind to?

Human viruses prefer alpha-2,6-linked sialic acids in the upper airway; bird viruses prefer alpha-2,3 links.

What is the most common symptom of influenza?

Onset is sudden, and the illness is more severe than a cold but less likely to include a runny nose.

Which two bacteria most often cause secondary pneumonia after flu?

About a third of primary viral pneumonias are followed by a bacterial one.

Roughly what share of the population catches influenza in a typical year?

That produces 3 to 5 million severe cases and up to 650,000 respiratory deaths worldwide.

Up to how many respiratory deaths does influenza cause globally each year?

Deaths fall mostly on young children, the elderly and people with chronic conditions.

Which age group is considered the most efficient spreader of influenza?

Children with fewer past exposures shed more virus for longer.

For how long is a person with flu usually infectious?

Healthy adults shed virus for up to three to five days; children and the immunocompromised for weeks.

Most flu vaccines are manufactured by growing the virus in what?

The process takes six to eight months, which is why strains must be chosen well in advance.

How often does the WHO meet to choose flu vaccine strains?

Northern and southern hemisphere flu seasons fall at different times of year.

How are live attenuated influenza vaccines given?

Most flu vaccines are inactivated and injected intramuscularly.

How many strains do trivalent and quadrivalent flu vaccines cover?

An H1N1, an H3N2 and one or two influenza B strains.

Oseltamivir must be taken within how many hours of symptom onset to be recommended?

It treats and prevents influenza A and B, and is also used to prevent infection in those at high risk.

Under what brand name is oseltamivir sold?

Zanamivir, its inhaled cousin, is Relenza.

Oseltamivir synthesis starts from shikimic acid, originally extracted from which plant?

By 2006 about 30 percent of the supply was being made recombinantly in E. coli.

What was the first antiviral drug approved against influenza, in 1966?

Newer antivirals came into use from the 1990s.

Which bacterium was wrongly identified as the cause of influenza in 1892?

R. F. J. Pfeiffer's mistake lives on in the bacterium's name.

In which year was influenza A virus identified as the cause of human flu?

Richard Shope had shown a virus caused swine flu two years earlier; influenza B was found in 1940 and C in 1947.

In which year was the first influenza vaccine licensed in America?

The US government developed inactivated vaccines during the Second World War.

In which year was the first convincing influenza pandemic recorded?

It began in East Asia and spread to North Africa and Europe; further pandemics followed in 1557 and 1580.

How many flu pandemics have occurred since 1900?

Spanish flu 1918, Asian flu 1957, Hong Kong flu 1968, Russian flu 1977 and swine flu 2009.

In which country did the 1918 Spanish flu pandemic likely begin?

It spread worldwide with soldiers during and after the First World War; a third to half of humanity was infected.

Which subtype caused the 1957 Asian flu pandemic?

It began in China and killed probably more than a million people, mostly the very young and very old.

Which American microbiologist first spotted the 1957 pandemic from newspaper photos?

He sent for samples, which reached the Walter Reed Army Institute of Research by mid-May 1957.

The 1968 Hong Kong flu was the first pandemic to spread significantly by what?

The novel H3N2 strain killed between 500,000 and two million people.

How is the 1977 reappearance of H1N1 thought to have happened?

The strain matched pre-1957 viruses so closely that a lab accident is suspected; it caused a pseudo-pandemic.

Where was the first human case of the 2009 swine flu pandemic identified?

The rural Veracruz town was ground zero for a triple-reassortant H1N1 that then mixed with a Eurasian pig virus.

What was unusual about who the 2009 H1N1 pandemic virus infected?

Older adults may have carried some immunity from earlier H1N1 strains.

Highly pathogenic H5N1 was first detected in 1996 in which animals?

It emerged in Hong Kong poultry a year later and caused a small human outbreak in 1997.

How is an avian flu strain classified as 'highly pathogenic' in the classic test?

The system dates from 1981 and says nothing about how dangerous the strain is to people.

Which avian subtype began infecting people in China in 2013, starting in Shanghai?

Some strains later mutated from low to high pathogenicity.

Italian phrases behind the word influenza blamed the disease on the influence of what?

Influenza di stelle reflected the astrological explanations of the 14th century.

The word influenza entered English during an outbreak in which year?

The shortened 'flu' is first attested as 'flue' in 1839.

What is the French-derived alternative name for influenza?

Other old names include epidemic catarrh and sweating sickness.

In 1648 it was discovered that which animals also get influenza?

Avian flu, then called fowl plague, was recognised in 1878.

What is the exaggerated immune response that can accompany severe flu pneumonia called?

H5N1 and certain other avian strains produce especially high levels of pro-inflammatory cytokines in the lungs.

Influenza D virus was discovered in 2011 in pigs in which US state?

Cattle were later identified as its primary reservoir.