THE TÁNAISTE LEO Varadkar said the coronavirus is “partially seasonal” earlier this week.

According to latest research and a Trinity College Dublin virologist, it’s too soon to make this determination, but it could be true after more time has passed and more research has been conducted.

The World Health Organization (WHO) has not said that the coronavirus is seasonal. 

So was Leo Varadkar wrong to make the ‘partial’ claim? Let’s take a look.  

The claim

The Tánaiste made the claim on RTÉ’s Claire Byrne Live programme on 25 January as he spoke about a potential easing of restrictions later in the year. 

“If we can get to the better weather, and this is a partially seasonal virus, so that does matter too, and if we can get a critical mass of people vaccinated, we can ease restrictions into that kind of Easter/summer period,” Varadkar said. 

The question of whether SARS-CoV-2 (the virus that causes Covid-19) is seasonal has been raised several times in the pandemic. 

What is a ‘seasonal virus’?

‘Seasonal’ refers to the fluctuation in virus transmission rates during different seasons of the year, leading to rates of infection either increasing or dropping.

These infection rates are impacted by a number of factors including climate and human behaviour.

The flu is one example of a seasonal virus, with the flu season lasting from October until around April in Ireland. 

SARS-CoV-2 is still a relatively new virus, so there are plenty of factors like human behaviour, transmissibility and climate to consider in deciding if it is seasonal. 

A spokesperson for the World Health Organization said that “to date, WHO has not said SARS-CoV-2 is a seasonal virus”. 

The WHO’s Dr Margaret Harris said in July that the season “does not seem to be affecting the transmission of the virus”. 

“What is affecting the transmission of this virus is mass gatherings, it’s people coming together and people not social distancing, not taking the precautions to ensure they are not in close contact,” Harris said  

“There seems to be this fixed idea about the virus being seasonal, but there is a huge outbreak, the most intense, the highest numbers, [which] are being experienced in the USA [and] they are in the middle of summer.” 

Obviously, the situation has changed significantly since then with the US experiencing even higher numbers throughout the winter season. 

 What is known for sure 

Dr Kim Roberts, a virologist in Trinity College Dublin, told TheJournal.ie that it is common for respiratory viruses to “show seasonality in temperate climates” in the northern and southern hemispheres.

“There’s lots of factors that might play a role in why that is. Part of it is down to human behaviour – we tend to be indoors more in the winter. But there’s also the fact that virus transmission can be affected by temperature and humidity,” Roberts said.

“But that’s not to say that transmission only occurs during that time. We know that respiratory viruses can transmit during other times of the year. It’s just that for many, once they become established in a population, they can have the seasonal pattern.” 

This happened after the 1918 flu pandemic, with the H1N1 flu strain remaining as a seasonal virus.

“It took that [virus] 18 months to two years to see that sort of pattern and I suspect that might be what happens with SARS-CoV-2,” Roberts said.  

She said Varadkar’s claim is not fully incorrect, but “we don’t have all the information yet to predict what’s going to happen in the summer”. 

“One theory is that we see seasonality in respiratory viruses like colds and flu because in the summer, we’re outdoors more and it’s harder for viruses to transmit.”

But she said that viruses can become better at transmitting, and also some viruses are able to transmit “quite efficiently” outdoors.

“It’s complicated. There are lots of things that can affect how well we control transmission. But, you know, we can be hopeful that over the summer, it will be harder for transmission to take place. And so restrictions can ease, by virtue of the weather being better, and all that,” Roberts said. 

But there are lots of other factors that play a role as well and so it’s likely that until significant portions of the world’s population are vaccinated, we’re going to be struggling for a while longer with this virus. 

She said she believes it is “likely that it probably will be seasonal”, but that it would “take another year” to determine. 

“This is the first time we’ve had a pandemic respiratory virus that we have known so much about so quickly, so we expect to know everything about it and also this is the first time we’re adding in vaccines very early on.

“We don’t know how that’s going to alter any mutation of viruses and all the rest of it so we can hope that we’ll see more and more seasonality, but we might not see the benefits of that seasonality until next year.”

Tánaiste response

A spokesperson for the Tánaiste said that although it is soon to definitively say the virus is seasonal, a number of scientific papers and studies indicate that it is. 

“This has been suggested by experts like Prof Paul Moynagh. Other coronaviruses and respiratory viruses tend to spread more easily in the winter,” the spokesperson said. 

“While it is too soon to say so definitively, there is evidence. If true, it’s good news as it will make it easier for us to keep levels low across the summer while we ramp up the vaccine. This doesn’t mean, however, that outbreaks cannot occur in the summer.”

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The spokesperson referenced a number of studies and scientific journals on this topic. 

They cited a scientific paper published in November last year which said it is unknown whether summer weather reduces the spread of Covid-19, but found that UV light is associated with decreased disease growth rate. 

This study made a prediction that Covid-19 would “decrease temporarily during summer, rebound in autumn, and peak next winter” in 2021. 

They also referenced a press release of a study conducted in the University of Sydney in Australia last June, which found a link between lower humidity and an increase in locally acquired positive cases of Covid-19 in the southern hemisphere.

However, this report says that in the northern hemisphere “there might be a risk even during the summer months” due to lower humidity levels. 

They cited a final report from the science journal Nature from October which said there is “growing evidence” to suggest that a small seasonal effect will “probably contribute to bigger outbreaks in winter”.

“But even if there is a small seasonal effect, the main driver of increased spread will be the vast number of people who are still susceptible to infection,” this report said. 

Research from the University of Illinois published earlier this week suggested that Covid-19 is seasonal according to global analysis.

This paper said that Covid-19 cases and deaths are “significantly correlated” with temperature and latitude across 221 countries. 

However, it also said that “more research is needed” to explain the role of climate and seasonality in Covid-19 incidences. 

More time needed

While some of these papers and reports indicate seasonality, Dr Kim Roberts said it would likely take more than one year to determine whether this virus is seasonal for a number of factors including vaccination levels among the population. 

She said the new, more transmissible variants like the UK strain also impact any determination of seasonality. 

“If it’s easier for SARS-CoV-2 to transmit between people [under new strains], we might start noticing more transmission outdoors, for example.

“And if there’s more transmission outdoors, then you might assume there’s slightly less of an effect from being indoors in wintertime because it’s cold and rainy. But again, we just don’t know yet so there are lots of factors playing a role.

“I think we’ll only find out if it’s seasonal once a reasonable proportion of people have already been exposed and not everybody is susceptible, because when you think about it,  it’s about how easy or how difficult transmission is.”

So while Leo Varadkar’s “partially seasonal” claim about the coronavirus may turn out to be true, it is currently too soon to say the virus is seasonal and we don’t know how it will react in the future as more people are vaccinated and new strains emerge.

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