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More severe flu season expected, but not yet back to levels before Covid-19

The influenza virus is usually imported from the northern hemisphere and starts circulating in SA around May/June. Stock photo.
The influenza virus is usually imported from the northern hemisphere and starts circulating in SA around May/June. Stock photo.
Image: 123RF/tommaso79

Health experts expect this year’s flu season to be far less severe than seasons before the Covid-19 pandemic, but more severe than last year.

The National Institute for Communicable Diseases (NICD) said reduced transmission of the influenza virus over the past two years has led to waning immunity against flu, and with the relaxation of Covid-19 restrictions it is likely there will be more flu cases this year compared to 2021.

However, more cases does not necessarily mean an immediate return to levels before Covid-19.

Shabir Madhi, professor of vaccinology at the University of the Witwatersrand, said  though it is difficult to accurately predict what will happen during this flu season “we can take some leads from what is happening in the northern hemisphere”.

He said the flu season in the US, which is tailing off, has been substantially less severe than in pre-pandemic seasons. The US Centers for Disease Control and Prevention reported that in the 2021 to 2022 flu season, 2,000 to 5,800 deaths occurred and between 3.5-million and 5.8-million people had flu illnesses.

This is a massive drop from the 2019 to 2020 flu season, which caused between 39-million and 56-million flu illnesses, and 24,000 to 62,000 deaths.

Madhi said the reason for the massive drop in cases and deaths in the US is unknown.

“It might be related to limited mobility of people, particularly intercontinental travel. This is an important feature in terms of recirculation.”

Added to this, Madhi said physical distancing, the wearing of masks and other interventions would have assisted in interrupting flu transmission.

It’s difficult to predict but we will probably see a worse flu season compared to 2020 to 2021 when we virtually had no flu. However, I feel optimistic we will have a relatively modest flu season compared to seasons before Covid-19
Prof Shabir Madhi

Responding to written questions, Prof Cheryl Cohen, head of the NICD Centre for Respiratory Diseases and Meningitis, and Dr Sibongile Walaza, an epidemiologist at the centre, said: “Most flu cases are never tested and diagnosed so we do not know the full numbers in SA. We do surveillance at specific sites and flu circulation in 2020 and 2021 has been lower compared to previous years.”

The influenza virus is usually imported from the northern hemisphere and starts circulating in SA around May/June. However, it can subside and come back around August.

“There is less chance of the flu virus being imported into SA with less travel. As human mobility increases, there is likely to be an uptick in flu cases,” Mahdi said.

Importance of vaccination

The NICD recommended people at high risk of getting severe influenza illness or complications should take the flu vaccine. It said the flu jab remains the primary means to prevent influenza infection and reduce risk of severe illness. The NICD website advises that to prevent being infected with flu, you should wash your hands frequently with soap and water or an alcohol-based hand rub and avoid contact with sick people.

Groups targeted in the department of health’s 2022 influenza vaccination campaign are:

  • healthcare workers;
  • individuals aged 65 or older;
  • individuals with cardiovascular disease (including chronic heart disease, hypertension, stroke and diabetes);
  • those with chronic lung disease (including asthma and chronic obstructive pulmonary disease);
  • those living with HIV; and
  • pregnant women.

With fewer flu transmissions during the Covid-19 pandemic, immunity to the flu virus will have waned, which is why it is essential immunocompromised individuals get the flu vaccine, said Madhi.

There are an estimated 17-million people in SA who are considered to be priority groups for vaccination.

“However, fewer than 10% of these people get vaccinated because not enough vaccine is procured and there is not a culture of people taking up flu vaccines,” said  Madhi.

Matching vaccines to flu strains

“In addition to the immunity gap, it’s also extremely difficult to predict the characteristics of the next strain that circulates,” Madhi explained.

“That’s one of the reasons the flu vaccine used in the northern hemisphere has had almost no effectiveness against the current flu virus because it’s a mismatch between what is circulating and what is included in the vaccines.”

In the northern hemisphere, where the flu season is coming to an end, the flu vaccines performed “poorly”, according to Madhi.

Influenza mutates frequently and the efficacy of flu vaccines are dependent on how well they protect against the circulating virus strains. New flu vaccines have to be made every year to reflect the mutations and different strains of the virus around the world.

This year’s strains and vaccines

Of the flu strains detected in SA, influenza A, particularly, and B are the most transmissible. Through the NICD influenza monitoring system, the influenza virus strains identified to date have been the usual seasonal influenza B strain with the Victoria and Yamagata lineages and influenza A with subtypes H3N2 and H1N1.

According to Cohen and Walaza, the trivalent vaccine covers all three important types of influenza. In addition to the strains this covers, the quadrivalent vaccine also covers the B/Yamagata strain.

In 2022, the trivalent vaccine is available in the public sector and the quadrivalent vaccine in the private and public sectors. The vaccines are free in the public sector to high-risk individuals.

Updated guidelines are that Covid-19 vaccines can be administered during the same visit as a flu jab. Opposite arms should be used for the two vaccines.

How well do flu vaccines work?

Cohen and Walaza agreed with Mahdi that flu vaccine performance varies each year, depending on how well the strain in the vaccine is matched to the strain which circulates.

Most countries in the northern hemisphere are using the quadrivalent vaccine, with the H3N2 strain dominating their flu season. Genetic drift within the strain itself determines how well the vaccines work.

Mahdi said: “The reality is that the quadrivalent vaccine has very little value and more so since the pandemic, where the Yamagata lineage of the B strain has largely disappeared.”

He said the inclusion of the Yamagata and Victoria lineages in the vaccine has “limited public health value”, had more to do with “marketing than anything else” and “doubled the price of the vaccine”.

Mahdi was “pessimistic”, given the experience in the northern hemisphere, that the vaccines will do better in SA this year.

“Fortunately, it’s going to be offset by us having a relatively attenuated flu season again this year,” he said.

“It can be anything between 10% and 80%. Unfortunately it is currently sitting closer to 10% than 80%,” he said regarding the efficacy of the vaccine.

Respiratory syncytial virus

SA is experiencing its worst outbreak in 10 years of respiratory syncytial virus (RSV), which is the most common cause of bronchiolitis and lower respiratory tract illness among young children. It is highly contagious. Infection with RSV does not result in permanent or long-term immunity and reinfections can occur.

This year has been a particularly severe season. At the moment the positivity rate for respiratory syncytial virus in children  admitted to Chris Hani Baragwanath Hospital is 60%
Prof Shabir Madhi

According to the NICD, in 2022 to date RSV has been detected in 19% (105/529) of children aged under five years and  hospitalised with lower respiratory tract illness at sentinel pneumonia surveillance sites. The details are summarised in the weekly respiratory pathogens surveillance report.

The number testing positive for RSV among children aged under five years started to increase in week 5 (ending February 13) and continues to increase, with 35% (28/81) and 32% (24/74) of children hospitalised with LRTI at sentinel sites in week 9 (ending March 6) and 10 (ending March 13) testing RSV-positive, respectively.

This article was first published by Spotlight.

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