Is it safe to have a Covid booster and a flu vaccine on the same day, doctor?

“I really believe this is why God gave us two arms — one for the flu shot and the other one for the Covid shot.” — Ashish Jha, White House Covid coordinator, September 6th, 2022.

Whatever this may say about the ability to predict the future, I suspect Gold had more functional aspects of two handedness in mind, when drawing up the prototype human being, rather than Dr Jha’s overly exuberant public health interpretation of divine motives.

But it does tie in with a concern expressed to me in recent weeks by readers and patients.

Is it safe to receive both vaccines at a single visit to your GP?

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Giving both the influenza vaccine and a Covid booster simultaneously, (with one shot into each arm), is official policy of the World Health Organisation (WHO) and the European Centre for Disease Prevention and Control (ECDC). Current clinical guidance states that Covid-19 vaccines may be administered without regard to timing of other vaccines, including the simultaneous administration of Covid-19 vaccine and other vaccines.

The thinking is that contaminant administration of the Covid-19 vaccine booster and the seasonal influenza vaccine will increase uptake of both vaccines. And vaccine co-administration would also be more efficient in an already overstretched health system.

But the most important consideration, and the one that has led to people’s concerns, is how safe is it to administer the two vaccines simultaneously? There is no evidence of interference with each vaccine’s usual efficacy. A 2021 UK study found that giving an influenza vaccine at the same time as a second dose of a Covid-19 vaccine produced no safety concerns and preserved the immune response to both vaccines.

However, a study of almost one million people, published in JAMA Network Open during the summer, found that the simultaneous administration of a Covid-19 mRNA booster dose and an influenza vaccine was associated with up to an 11 per cent increase in systemic reaction compared with having the Covid booster alone.

The systemic reactions (distinct from local injection site effects such as bruising) reported by participants in the week following the double immunisation included fatigue, headache, and muscle pain. While statistically significant, the increase in reactivity is small: some 8 per cent among those who received the Pfizer mRNA booster and 11 per cent among those who received the Moderna mRNA booster.

In addition, those who received the Moderna booster and the flu vaccine were more likely to report a health impact such as being unable to work, attend school, or perform normal activities.

Another factor, which readers may wish to consider when timing their influenza vaccine is the expectation for flu to mount a major comeback this year. The flu season is just coming to an end in the Southern Hemisphere: cases in Australia started to increase in April, two months earlier than usual, before peaking in early June. It is the country’s worst flu season in five years, especially among young children. Some South American countries experienced more significant flu seasons than the past several years too. Most infections were caused by the influenza A virus H3N2. Early Antipodean estimates point to a 2022 vaccine effectiveness of about 40 per cent. This is an important level of protection, with hospitalisation rates among vaccinated people typically much lower than in unvaccinated populations.

And while the northern hemisphere doesn’t always follow flu patterns from south of the equator, it may be prudent to arrange for early influenza immunisation this year. Although be aware also that the estimated effectiveness of the flu vaccine tends to decline by about 18 per cent for every 28-day period after immunisation.

So is it safe to take the flu and Covid booster vaccines on the same day?

The answer is yes, but be aware of a slight increase in adverse events compared with having the Covid-19 booster alone.

mhouston@irishtimes.com