H3N2 Virus Surge in India: Why Do Viruses Mutate?

FIT traces the history of the virus and explains how it has mutated over the years into different subtypes.

(Photo: Garima Sadhwani/FIT)

The first outbreak of the H3N2 virus was in 1968, then known as the Hong Kong Flu or the Flu Pandemic. Now in 2023, H3N2 virus is being attributed as the cause behind surge of flu cases in India.

FIT traces the history of the virus and explains how it has mutated over the years into different subtypes.

The First Appearance: Pandemic of 1968

The pandemic in 1968, caused by influenza A, resulted in over a million deaths globally. According to the Centers for Disease Control and Prevention, the virus was made up of two genes – both from an avian influenza A virus – a new H3 hemagglutinin and the N2 neuraminidase. 

The H3N2 virus could be traced back to the H2N2 virus through what is called an antigenic shift. The H2N2 had previously been a pandemic in 1957-58. The hemagglutinin antigen of the H2N2 virus genetically mutated to produce the H3 antigen.

But since the virus retained the N2 antigen, against which people had some level of immunity, it was milder and caused fewer deaths, affecting mostly the elderly. But it was still highly contagious and spread quickly worldwide. 

The symptoms of contracting the H3N2 virus have remained common over the years. In fact, the H3N2 virus behind the surge in cases in India recently has genetically evolved from the same virus that caused the pandemic in 1968. Its symptoms, which can last up to a week, include:

• Upper respiratory issues

• Chills

• Fever

• Muscle pain

• Weakness

Immunity Against H3N2 Through Infection & Vaccines

While people had started developing natural immunity against the virus, the first trivalent vaccine for influenza viruses came in 1978. It provided immunity against two strains of the influenza A virus and one strain of the influenza B virus.

Another quadrivalent vaccine, which provided immunity against two strains each of influenza A and B viruses, has also contributed to health security for the global population.

However, according to a study, titled H3N2 Influenza Viruses in Humans: Viral Mechanisms, Evolution, and Evaluation:

"Since their introduction to the human population in 1968, H3N2 Influenza viruses have rapidly evolved both genetically and antigenically in an attempt to escape host immune pressures."

Why Viruses Mutate

After the 1968 pandemic, the H3N2 virus has still led to many smaller outbreaks of infections in communities through different mutated subtypes. This brings us to the question of why viruses mutate.

Simply put, viruses mutate, thanks to evolution, for survival. 

Oftentimes, while trying to replicate a virus, the cells might accidentally change some or the other part of the viruses’ genetic material which might create a new stronger or weaker strain of the same virus.

The changes, more times than not, are in the genetic sequence of the viruses’ ribonucleic acid (RNA). Interestingly, though, viruses cannot mutate on their own. They need a host body to replicate their cells, which increases the rate of infection. 

The H3N2 virus has mutated many times over since 1968.

In fact, the H3N2 virus also had a swine flu variant. A report in ScienceDirect stated that "the H3N2 variant virus is a descendent of a lineage of a triple-reassortant H3N2 virus with human, avian, and swine segments that began widely circulating in North American pigs in 1997-1998."

Most recently, in 2011, after the H1N1 (or swine flu) outbreak, the H3N2 'variant' viruses "with the matrix (M) gene were detected in people," according to the CDC.

Another study, available on PubMed Central, titled Recent H3N2 Influenza Virus Clinical Isolates Rapidly Acquire Hemagglutinin or Neuraminidase Mutations When Propagated for Antigenic Analyses, stated:

"Since 1968, H3N2 viruses have steadily acquired mutations in the exposed globular head of HA. Many HA mutations simultaneously alter the antigenicity and receptor binding properties of influenza viruses. Recent H3N2 strains have dramatically reduced receptor binding avidity and altered receptor specificity."

What You Can Do

While these mutations make the virus sound very scary, there are some simple things you can do to protect yourself.

• Wash your hands frequently

• Wear masks in public

• Stay hydrated

• Don't shake hands with people

• Avoid touching your face

• Avoid self-medicating if you have any symptoms and consult a doctor