Increased pollen in the air related to increase in coronavirus infections

Published on
March 9, 2021

Global research has revealed that increased pollen in the air is associated with an increase in coronavirus infections. The SARS-CoV-2 virus may exploit the weakened immune response as a result of exposure to pollen. This is not only the case for hay fever patients, but also for people who are not allergic. From the Netherlands the Leiden University Medical Center (LUMC), the Elkerliek hospital and Wageningen University & Research (WUR) are involved in the study. The results were published in the scientific journal PNAS on March 9.

It was already known that exposure to pollen in the spring weakens the immune system’s response to viral respiratory infections, such as the cold virus. This effect occurs in people who are allergic to pollen as well as people who are not allergic. During the outbreak of the coronavirus, it was noted that in many countries the number of infections only began accelerating after the pollen concentration increased. Researchers hypothesised that an immune system that has been weakened due exposure to pollen may be more susceptible to the coronavirus.

Worldwide: increased pollen, increased infections

To examine this, a team of 150 scientists studied the pollen counts from 130 pollen stations in 31 countries, including the Netherlands. The pollen effect was apparent throughout the world at the start of the epidemic. The study shows that the number of infections increased after the pollen concentration rose four days earlier. The infection figures increased by an average of 4 percent after the pollen concentration increased by 100 pollen per cubic metre. The implementation of a lockdown halved this effect, but the amplifying influence of the pollen remained visible.

In the Netherlands as well?

This amplifying effect was also found in the Netherlands, but it declined after strict statistical testing. A variety of strict statistical tests were used to analyse the large amount of information and to determine whether there was a relationship between pollen and the coronavirus. This was a challenging analysis because many different factors determine the rate of infection. Furthermore, the data collection for this study ended after 8 April, which means that a large part of the spring pollen surge was not included in the analysis. This is partly why the connection between pollen and the coronavirus could not be clearly demonstrated for the Netherlands.

Pollen and the virus

The reason that the infection figures only increase four days after an increase in pollen exposure is because it takes a few days before the virus manifests itself and for people to get tested. This corresponds with an earlier study which showed a coronavirus incubation time of 4 to 5 days. Researchers emphasise that the positive correlation between pollen and the coronavirus is not the result of pollen spreading the virus more effectively, but of a weakened immune response due to pollen. The virus is transmitted through contact between people.

Follow-up research required

The article suggests that exposure to pollen contributes to the increase in the coronavirus infection figures in many countries. Whether this also applies to the Dutch situation must be investigated further. Research must also confirm whether the relationship between the two is causal. International studies are being set up in which lung cells will be exposed to pollen and the coronavirus SARS-CoV-2. This type of research is much more suitable for determining whether exposure to pollen actually weakens the body’s immune response to the virus.

Pollen situation in the Netherlands

After the short frost period, the alder pollen concentration in the Netherlands increased rapidly from 18 February. On Tuesday 23 and Wednesday 24 February, the LUMC and the Elkerliek hospital registered a very high peak of around 1,400 pollen per cubic meter of air, after which the concentration decreased rapidly again. In addition to alder pollen, the concentration of pollen from the Cypress family / Taxus and poplar also rose remarkably high. That concentration also dropped again after a few days. Whether the appearance of the pollen in the air contributed to the resurgence of corona infections around that time remains to be examined. After a somewhat cooler period, the temperature will rise again in the coming week, but the probability of precipitation will also increase. When it rains, pollen washes out of the air. In addition to the persistent flowering of alder, several trees gradually blossom during the course of March. Especially pollen concentrations of birch, ash and poplar can rise to relatively high concentrations. The final concentration will depend on a combination of factors and is currently still difficult to predict. Every week, the LUMC on Tuesday and the Elkerliek hospital on Wednesday publish the pollen count from the previous week.

Corona test

Hay fever symptoms may be mistaken for COVID-19 symptoms. However, hay fever does not usually cause a fever or a loss of smell, but those are symptoms of coronavirus. The government advice of “if you think you need to get tested, then get tested” also applies here.