The time when SARS-CoV-2 seemed to be the only respiratory virus around was long gone last season. This autumn, respiratory syncytial virus (RSV) is already overcrowding hospitals, while influenza is starting to make its presence felt.  

Amparo Larrauri, head of the Influenza and other Respiratory Virus Surveillance Group at the National Epidemiology Centre, and Francisco Pozo, senior scientist at the Reference Laboratory for Influenza and Respiratory Viruses at the National Microbiology Centre, both researchers at the Carlos III Health Institute, analysed the situation in a meeting with journalists organised by the Science Media Centre Spain. 

Explosion of bronchiolitis cases

Larrauri explained that the presentation of RSV is "totally usual", as it was in the previous season: "We had a peak in December 2021 and now the same thing is happening, at its usual time". What was unusual, he recalled, was the summer peak in 2021, when "respiratory viruses that had been "displaced" by SARS-CoV-2 "started to emerge again". 

"This year [SARS-CoV] is not particularly severe, it is comparable to the severity it has always had," Pozo said. "What happens is that [in pre-pandemic seasons] it always arrived in an explosive way with many cases at the same time, in a very explosive wave that was short and gave way to the flu". Both researchers clarified that it is still too early to tell whether the same thing will happen this year.

"There has been an epidemic wave of RSV somewhat earlier than usual, with hospitalisation rates, especially in younger children, increasing significantly week by week," Larrauri explained.  

Although this is generally observed in all autonomous communities, the scientist explained that comparison with previous seasons is complicated due to the new surveillance system, which until now only offered each week the percentage of positive samples (positivity) and not hospitalisation rates. 

The immune debt hypothesis

There is a hypothesis that this RSV season is being more important than usual because many children have not been exposed to respiratory viruses during the pandemic, so the susceptible population is larger. 

"It is a hypothesis that cannot be ruled out. For two years there was almost no flu circulating and it is logical that children who have not had a previous history of infections may be more affected," Larrauri said. Even so, he recalled that "there was already an epidemic [of RSV] in the summer of 2021" and that this "somehow influenced the immunity that these cohorts of children may have".

The [immune debt] hypothesis is more plausible in a population that has already had a lifetime history of infections

Francisco Pozo

"The only thing is that we are seeing it day by day and with an important intensity," Larrauri continued. Both researchers explained that they do not know when the peak of RSV will arrive, but that it has not yet been reached. 

"The [immune debt] hypothesis is more plausible in a population that has already had a history of infections throughout their lives," said Pozo. "With RSV we are talking about the vast majority of hospitalised cases being less than 2 years old, and little history of infection they may have, pandemic or no pandemic." With influenza, he continued, it may be different: "We have had two years without people being exposed and maybe now there will be an explosion of cases, but with RSV it is not so clear". 

Larrauri pointed out that RSV is especially important in children under one year of age: "Most of them were not even exposed to the [RSV] wave of spring 21". 

What about the flu?

Pozo recalled the unusual nature of last flu season: "It had two peaks, with a second one in spring-summer which surprised us a lot, it was the first time we had seen it". Larrauri added that the flu did not stop circulating throughout the summer "at identifiable levels" until "it started to increase at the end of September". 

Australia already had an early flu season and Europe and North America are seeing the same. "We are already seeing an early presentation, with incidences of influenza in primary care already above the baseline threshold," Larrauri said. "We are in a flu epidemic period. Hopefully the peak of RSV will come before the peak of influenza, but at the moment both are there," he added.

We are in a flu epidemic period. Hopefully the peak of RSV will come before the peak of influenza, but at the moment both are there

Amparo Larrauri

The researchers made it clear that it is not possible to know what this flu season will be like and whether there will again be two waves as in the previous one. Larrauri did clarify that there are seasons that have been earlier than others, such as 2003-2004 and the 2009-2010 pandemic. 

As for the effectiveness of this year's flu vaccines, Pozo said that "for the moment there is not much change", so they expect there to be similarities between the strain circulating in the majority in Spain (H3) and that of the vaccines used this year. 

"In February the first analyses [on the effectiveness of this year's flu vaccine] will come out, it is possible that it will give slightly better results than other years," added Larrauri. "In any case, the vaccine always gives better results against H1N1 and B than against H3. 

The researcher also highlighted the "wisdom" of bringing forward the start of the flu vaccination this year, given the lack of knowledge of what was going to happen and the possibility that the epidemic would start earlier. 

SARS-CoV-2, destined to co-exist with other respiratory viruses

"The three viruses already coexist [since last season] and will continue to do so. It is the addition of one more respiratory virus and it is something we will have to get used to," Pozo said. 

"Depending on the transmissibility of a virus, epidemic waves present themselves in one way or another," Larrauri said. "Influenza began to appear [in 2021] in its usual period, December, and had a peak and dropped immediately when a new and tremendously transmissible variant began, such as omicron. It was just when the omicron wave began to subside that the flu came back and gave rise to that second epidemic wave of flu that was later, more intense, very prolonged and outside its usual period," he added.

SARS-CoV-2 is normalising and allowing other [respiratory viruses] to circulate

Amparo Larrauri

The researchers explained that the emergence of a new variant with a special transmissibility may influence the rest of the respiratory viruses and that there is still uncertainty about this. Even so, Larrauri considered that "SARS-CoV-2 is normalising and allowing the others to circulate. There is always a balance of ecological niches, but in principle all winters are full of respiratory viruses circulating at the same time, some earlier and others later. 

"For covid-19 at the moment we do not have a significant increase in positivity and there seems to be a certain stabilisation at the national level, but there is a certain tendency for hospitalisation rates to increase in some territories," Larrauri said. "We can't say anything yet because we don't see a clear trend," he stressed. 

The importance of surveillance systems

Larrauri stressed the importance of surveillance systems to know the levels of circulation of respiratory viruses in the community. 

"All the information we are providing is obtained from the new SiVIRA surveillance system for acute respiratory infection, both in primary care and in hospitals," he said. "Samples are taken from symptomatic people who come to certain health centres, called sentinels. A triple diagnosis is made by PCR of influenza, SARS-CoV-2 and RSV, so that the positivity is telling you the true circulation of these viruses in the community," he explained.  

In other words, "an estimate of what there is at a national level and in each autonomous community" is obtained to give "a real vision of what is happening", he concluded.